A friend of mine had a really funny idea of putting little speakers in the bushes near the dumpsters.
While they dig through the trash, you can whisper stuff to them

If someone is so sleep deprived and twacked out of their gourd they would already see and hear stuff that isn't there, by giving them the idea that the garbage has a spirit which is talking to them....possibly warning them that my apartment complex is haunted by transexual nazi eskimos...they might think twice about coming back :-)

I was thinking you can take the idea even further by adding little sound effects such as R2-D2, a crowd cheering, the sound that Fred Flintstone makes when he's running etc etc etc