Start by going to the graveyard. Once there, click all the tombstones. With any of them that say "Pick Up" or "Take", do so. Once you are sure you have all the tombstones you can get, return to your house. Place them outside somewhere in your backyard. You now have a 70% chance of a ghost or two, or even a whole party of ghosts visiting your house each night. To improve your chances, write in the epitaph "Here Lies [Sim's name], The One Who Returns".