From Nookipedia, the Animal Crossing wiki
 In Animal Crossing
Each Animal Crossing town starts with six villagers. Over time, more move in until the maximum of 15 is reached. Once this occurs the game will cycle out the 14th or 15th villager every ten days or so while keeping the initial 13 villagers intact. A villager may mention moving in a random conversation and ask the player for an opinion, but the player's response does not affect villager movement. A moving villager will not pack up prior to leaving; their home will simply be removed from the map upon game load and a goodbye letter is sent to the player's mailbox. Unlike subsequent Animal Crossing titles, once a villager has moved out another usually replaces them immediately.
 Movement across memory cards
Players are able to travel to other Animal Crossing towns by speaking to Porter at the Train Station. Each time a player goes to visit another town, one of the traveling player's villagers is chosen at random to move out, and anytime a town is visited by another player, that town will receive the resident from the visiting player's town. This is the only way to cycle out one of the 13 original villagers.
 Controlling villager movement
By taking advantage of the way villagers move across memory cards, the player can manipulate which villagers come and go. In order to do this, three memory cards are required: one with the primary town's Animal Crossing data, a second with space for travel data, and a third with a secondary town that will be used as a repository for unwanted villagers.
 Removing unwanted villagers
Because villagers are selected at random to move when visiting another town, moving out a specific villager is a matter of trial and error. To begin the process, follow these steps: (the town must have seven or more villagers in order for this to work)
This same process can be used in order to move in a desired villager from one town to another.
 In Dōbutsu no Mori e+
The E-Reader features of Animal Crossing are expanded upon in Dōbutsu no Mori e+ with the addition of 60 new villager cards as well as 18 new islanders. In addition to providing the player with items, scanning these cards will move the character featured on the card into the player's town. This feature is backwards compatible with the E-Reader cards released for Dōbutsu no Mori +, giving the player an additional 273 villagers to choose from.
The game also includes an island for each playable character, however unlike Animal Crossing each island does not come with an islander by default. Instead, the player must scan one of the islander e-cards and the islander will wash up on the island's shore (just like Gulliver) the next day.
 In Animal Crossing: Wild World
In Animal Crossing: Wild World, villagers attempt to move by packing their furniture up in boxes. While the villager is packing the player can convince him or her to stay, although it may take several tries. If the villager is not convinced to stay they will move away, leaving a signpost behind and sending a goodbye letter to the player's mailbox. Another villager (usually of the same gender) will move in to replace that villager within a week. The game will attempt move a villager out every two days; this causes the villager to forget their current hobby and switch to a different one if they are convinced to stay.
 The move-in process
Due to Animal Crossing: Wild World's small town size, there are only three villagers present at the start of the game. Over time more villagers will move in until the town reaches its maximum of eight villagers.
 The ninth slot-in
Within the game's code exists a space used to store incoming villager data received from other Wild World towns via WiFi or DS to DS local connection; this is referred to by players as the 'ninth slot-in'. It can be thought of as a waiting room where a villager received from another town will sit until there is space for them to move in. This villager will remain in the game's saved data indefinitely until it can move in or is replaced by another villager. To override the villager in the ninth slot-in the player must visit or host another player over WiFi or DS to DS local connection that has a villager waiting in their ninth slot-out. The villager in the other player's ninth slot-out will transfer to the player's ninth slot-in.
Note: It is not known how the game handles data when interacting with more than one other player at a time.
 Moving in a specific villager
In order to move in a specific villager, three requirements must be met:
Even if the player does not have room in their town for an additional visitor, they can still receive the villager's data; it will simply override any existing data in their ninth slot-in. To receive the data, they must host or visit a player whose town contains ninth slot-out data for the villager the player wishes to receive.
 The move-out process
Once a town reaches its maximum villager limit the game will choose a random latent villager to move out of town. A latent villager is one who is in-between hobbies. The player can use this fact to their advantage and keep the unwanted villager in latency as often as possible (by completing their requests e.g. catching a bug, getting them a specific piece of furniture etc).
 The ninth slot-out
When a villager leaves town, their data is not immediately erased. Instead, it is stored in what is called the 'ninth slot-out', a space in the game's memory used to store villager data beyond the eighth villager. This data can then be exchanged with other players over WiFi or DS to DS local connection. In a case where two villagers move out one after another, the original villager's slot-out data would not be overridden. Some players theorize the second villager's data is deleted, while others believe it is stored in what might be called a 'tenth slot-out'- a backup slot used if the data from the ninth slot-out cannot be transferred to the receiving player. Example: Player A's ninth slot-out is Bob, but Player B already has Bob in their town and so the game gives Player B the data from Player A's tenth slot-out, Olivia.
It's also important to note that when interacting with other players, slot-out data always replaces slot-in data; it is not possible for players to exchange slot-out data for slot-out data or slot-in data for slot-in data. In cases where both players have slot-in and slot-out data, data is not exchanged at the same time, meaning that one player's slot-out data may override the other's slot-in data before a mutual exchange can occur.
Note: It is not known how the game handles data when interacting with more than one other player at a time.
 Clearing slot-out data
As previously noted, a player's slot-out data is not overridden when another villager moves out. It can be removed in one of two ways: by being deleted, or by being transferred to another player.
To delete the data, simply enter Tag Mode with no other DS's in range. This is useful when you want to assure that no one else will receive your character data, if for example it contains a glitched villager or a villager with an inapproproiate catchphrase.
Note: It is not known whether data in the tenth move-out slot is deleted via the Tag Mode method (if a tenth slot exists at all).
To transfer data to another player, simply visit or host another player over WiFi or DS to DS local connection. The only time data will not be transferred is if the villager already lives in the other player's town or if the data for that villager is in that player's move-out slot.
 In Animal Crossing: City Folk
In Animal Crossing: City Folk, moving works similarly to Animal Crossing: Wild World, but rather than immediately packing their furniture in boxes, moving villagers will approach the player while outside and ask them whether they should move away or not. They can be convinced to stay as before, although it can again take multiple tries. If a week passes and the villager is not convinced to stay, they will pack their belongings. Once this happens, they cannot be convinced to stay; they will move out after two days and will send the player their departure letter.
As in Animal Crossing, there are six villagers present at the start of the game, one of each personality. One villager will move in each day for the next three days. The villager maximum is ten and the final villager can take up to a week to move in.
 Frozen villagers
The concept of the 'ninth villager slot' from Wild World returns in City Folk, however instead of being described as occupying a hidden extra villager slot, villagers are said to be 'frozen' in the game's data after moving out. While the process for moving villagers in and out is identical to Wild World, it is complicated by the ability of villager data to be sent to other players via WiiConnect24. This feature allowed players to receive villager data from a town they had never visited, similar to the Spotpass feature in New Leaf. In order to maximize the chances of delivering or receiving villager data, it was recommended that the player turn off WiiConnect24 and remove all players from their friend list except the one they intended to WiFi with. WiiConnect24 was discontinued on June 28, 2013 with Nintendo Wi-Fi Connection terminating in July of the following year. As the Wii does not allow for local multiplayer play, players are no longer able to exchange villager data without using a replacement server host such as Wiimmfi.
 In Animal Crossing: New Leaf
In Animal Crossing: New Leaf, towns no longer feature permanent signposts marking possible villager plots, instead new villagers may move in anywhere there is space. The day before a new villager moves in a roped area of land will appear with a signpost noting the future villager's name. Additionally, villagers will now tell the player the date that they will pack up their furniture and after leaving, may send their picture along with their goodbye letter if the player has befriended them.
 The move-in process: A brand new town
Every town starts with just five villagers; their personalities will vary, as the game attempts to maintain a balance between the eight types. The day after a town is created, a new villager will begin the move-in process. An empty plot will appear, roped off with a sign in front. The following day, the villager's house will be constructed and they will be unpacking their boxes. Isabelle will make a note on the startup screen that someone has moved in.
On the fourth day, a new area will be roped off, ready for another villager to move in. This constant move-in process will continue until nine villagers are reached at which time no more villagers will move in without some help from the player.
 The tipping point: nine villagers in town
A town with eight villagers or less is considered to be in a ‘moving-in period’. During this time, no villagers will ever move out and the game will move in new residents at a constant pace. Once a town reaches nine villagers, it will naturally want to reduce the count back to eight, but is in no hurry to do it. It will be at least three days if not much longer before an animal moves out. If desired, one last villager can be added to the town to bring the total to ten. This can be done in one of four ways:
 Four ways to get a tenth villager
Note that any of these methods can be used to obtain a ninth villager as well, however the tenth can ONLY be obtained via these methods.
 Influencing move-ins
Resetting at the source: Players seeking out a specific villager have a few options, the first of which is to delete their town and start anew or acquire a second copy of the game. A randomized selection of five different animal residents will be generated each time a new town is created. This method will probably take the longest, as there are 333 different villagers total for the game to choose from.
If the player has the campsite public works project the player can cycle through villagers there as well by continuously creating a new character the day that a camper comes to visit (this requires foresight/a bit of luck).
Resetting by personality type: The game tries to maintain a balance of personality types, so when a new villager is generated it will be of a type that the town currently lacks or has the least of. This can be used to one's advantage; the player can reset their game until it produces a desired villager of the personality type it is currently generating. (e.g. if the player is looking for a specific uchi villager and the game is currently generating that personality type, there is a 1/21 chance of getting the player's preferred villager... that’s way better than 1/333!) Follow these instructions to properly reset the game:
 The villager reset trick
This trick will allow the player to cycle through possible move-in scenarios, and to pick the villager or house location that best suits their needs. In order to use this trick, the town must have less than four human characters created.
On a day when a new villager will (or is suspected to) put down a house plot, begin the game by creating a new character- do not load an existing character. After getting off the train, look around town with the new character to see if a new villager has set up a plot. Remember that a villager will only move in during a move-in period (when there are less than nine villagers in town) or if the player has fulfilled one of the conditions listed under the ‘Four ways to get a tenth villager’ heading.
If there is no house plot, quit the game (don’t save) and create another new character. After going through this process about three times with no luck, then a villager will most likely not move in that day. To be safe, the player should choose a home location for the newly created human character, then save and quit. The new character can then be deleted (if they are unwanted/unneeded). It's recommend that the player always place a newly created character's home in the same spot, as the area where their home used to be will turn to dirt once the character has been deleted.
If there is a house plot, look and see who it is. If it is a desired villager, and/or if the villager's home is properly placed, choose a home location for the new human character, save and quit. Otherwise, do not save, and keep creating a new character until a suitable villager and/or location is found. Take note of the personality of the villagers the town is generating, this will let the player know whether it is in their best interest to continue resetting, or if it’s not worth the time. It’s important to note that if a villager of a given personality type just left town, the next villager the town generates will never be of the same type. (e.g. If a smug villager just moved out, a new smug villager will never move in after them unless they are invited in/they are received from the void or SpotPass)
Note: The reason it is so important to create a new character when using this trick, rather than loading an existing character, is because of the nature in which the game chooses to save town data. When an existing character is selected, the game loads the town data and then it saves the game without any notification. That means that any villager plot seen while walking around town with a pre-existing character is permanently placed. Even if one were to quit without saving it will remain and that villager will move in the next day. There is no way to move a villager plot once this happens.
On the contrary, when a new character is created, the game loads the town, but does not save it until that new character has established a home and has finished registering with Isabelle. This allows the player to keep reloading the town data until an agreeable villager plot is generated.
 Villager home placement
New villagers seem to want to move in close to where other villager and human character homes are placed. If there is an area the player wants villagers to move into, they should try placing all human homes in that area. In addition to this, new villagers like to set up their homes near the spot where the most recent (and sometimes second most recent) move-out was living. It should also be noted that they can and will move in on top of paths, trees, bushes, flowers, and even dropped items. If the town has a police station these items will show up in the lost & found, otherwise they are gone for good.
A villager’s home will never brush up against another permanent, immoveable object (including the river). They require a buffer of one space to their left, right, and back, and a buffer of two spaces in front. Their home may be placed as close as two spaces away from the ledge separating the town from the beach and as close as one space away from the ledges on the Eastern and Western sides of town. Two homes can be as close as two spaces apart on the left and right sides and as close as three spaces apart on the top and bottom sides. They may be as close as one space apart from rocks, ponds, rivers or public works projects, but will never touch them diagonally. A villager’s home will never block access to a ramp or bridge.
 The move-out process
Once a town has reached nine or more villagers, it will be in a ‘moving-out period’. During this time, the game will continually try to move villagers out- no villagers will move in unless they are invited them from the campsite/another town or are acquired from WiFi-ing or SpotPass.
 Who’s moving next? (How does the game decide?)
The mechanics behind how the game choses the next villager to boot are currently unknown, but there are few observations. The most recent villager to move in will never be the next villager to move out. Additionally, it seems that villagers who have been in town the longest are more likely to leave than those who have recently moved into town, and ignoring villagers is not a surefire way to get them out either; talking to unwanted villagers may work better. The general rule is that it is random, and a player is most likely going to have to deny moving requests from villagers they want to keep before the one they want gone will leave.
 How to tell if a villager is going to leave
A villager will ‘decide’ to leave up to five days before they actually pack up. It is during this time that the player can convince them to stay, or tell them it’s ok to leave. There are two ways to determine who is thinking of moving:
However, it’s not as simple as this- a villager will only ping if the player is on ‘speaking terms’ with that villager. The player can determine if they are on speaking terms with a villager by walking up to them and speaking to them. If they start off with something like “I haven’t talked to you in a while!” then the player was not on speaking terms with them… but by talking to them just now- they are! After getting back onto speaking terms with a villager the player can save/quit and reload the game and walk in front of them to see if they will ping.
Note: If the gossip method was used to figure out who was leaving, the second method must still be used to convince the villager to stay or go. This is why it is recommend to skip the gossip and cut to the chase.
 The move-out date & letting go
Once a villager pings and informs the player they are moving, they will ask whether or not they should stay. If the player tells them to stay, they will not leave. However, this does not mean that they will never decide to move again sometime in the future. If the player says yes, they may either be excited and thank them for understanding, or they may change their mind and stay anyway.
If they change their mind and decide to stay, but the player really wanted them gone, the player can quit their game without saving and the villager will move out on the day they stated they would.
Additionally, if a villager decides to move and the player does not tell them to say, they can change their mind later on and decide to stay. They might reveal their change of heart sometime during conversation or even without telling anyone. To avoid this, the player can time travel to the villager's move-out date, thus ensuring that they will move out on time.
Note: A villager’s ‘moving date’ is the day they will be inside their home with their boxes packed. If a player wants a villager to stay, they have to convince them to do so prior to this date. If the game is loaded on their move-out date and Isabelle announces they are leaving, they cannot be convinced to stay; the player has passed the point of no return. This would be the day where the player could have someone come over and adopt them.
 Villager cycling
Villager cycling is a term used to describe a fast and efficient method of generating move-out requests in order to remove a specific villager from town in the shortest amount of time. There are multiple methods used to cycle villagers, all of which involve time travel. Start the process by following these steps:
Note: Before beginning it is recommended that the player set their town to the beautiful town ordinance to prevent flowers from wilting and weeds from spawning. Also note that the town will be overrun with common flowers and gyroids if this method is used often.
 Getting back a long lost villager
Worst case scenario... the player just loaded up their game and Isabelle says that one of their dream villagers is leaving town today- what can the player do? They can't be convinced to stay, the only thing that can be done is to have a friend come over, adopt them and hold onto them until the player can get them back. But it’s not as simple as inviting them back over- they won't come back until the player has gone through...
 The 16-villager cycle
After a villager leaves town (whether they are adopted or sent to the void), their data is still stored in the game’s memory to allow them to visit the town to shop on Main Street. The player will be unable to reacquire that villager until this data has been overridden.
Once a dream villager has been sent off to a friend for safe keeping, the player should start keeping track of how many villagers have been cycled out of town. The villager cycling method outlined above can be used to move villagers out as quickly as possible. After 16 more villagers have left town, the player will be able to go and pick their dream villager up again.
 Keeping dream villagers in place
The easiest way to make sure everyone stays put is to never allow the date to change. Prior to loading the game, set the system’s internal clock back to the last date played. No villagers will ever leave. If a player is going to use this method they have to remember that the game recognizes the start of a new day at 6AM, not midnight, so they need to make sure the game is not powered on and loaded up when 6AM rolls around.
 Going on hiatus
To make sure that no villagers will have moved upon returning, follow these steps:
Before going on hiatus
Coming back from hiatus - The following must be done before loading the game!
If the game was loaded on the SAME DAY that the player last played, no villager loss should be experienced. If the game was loaded to anytime BEFORE that day, it is possible for a villager to move if they had already planned on moving.
Getting back to the present
Once the player has denied a request to move they are free to time travel forward as far as they'd like. So, after saving and quitting the game, go back to system settings and correct the date/time. Then, load the game and the player will be back to the present.