It's funny how many people can still act like children after becoming adults. If that person is your roommate, it's not that funny.
Before signing a lease, have a sit down, serious talk with your potential roomie. A serious conversation before he or she moves in with you should cover your lifestyles, monetary stability, dating habits and cleaning habits to see if they're compatible.
Keep the conversation honest, open and open-ended. Maybe neither of you is in a current relationship, but in 3 months when you keep coming home to find Roomie's boyfriend asleep on the couch in the middle of the day, it's time to revisit the discussion.
Sometimes no matter what, horrors happen. Because sometimes personalities simply clash. Or hygienes don't align. Everyone's got a horror or humor story to tell about an ex-roommate. The best advice we have is to try to see it all as something that will be humorous in the long term. You can always call the landlord or the cops, if it's serious or dangerous in the short term.
When nature calls and you can't use the bathroom because your she-devil of a roommate refuses to relinquish the mirror because she's still doing her make-up...or when your cable or phone bill is $200 more than normal, with charges to 976-LOVE, then you'll understand what we mean. And that it's time to break up your partnership, and either find a new roomie or a new place to live.