Basically when you're still having villagers move in & out, you put tiles on the spaces where you don't want them to move onto. That way, you can pretty much decide exactly where they are putting their house! That's how some players have their villager's houses lined up perfectly, without doing any hacking.
The plot resetting part is that when you know someone's moving in, you start up a new Save File in your town. That way you can look around your town to check whether they moved into a spot you like (which should be easy when you have your tiles spread around town to reduce the spots they can actually move into. When they move into a bad spot, just turn off your DS and start this process again. When they have arrived in a good spot, you can finish creating the new Save File until you can save. Then, just delete the new Save File and start up your game like normal again!
I hope that made sense