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".