make sure anyone who lives there is on the lease. If they want move and you want to stay they need to find someone who will take thier place on the lease, unless you want the place all to yourself.
I watch a lot of Judge Judy... Also, make sure that you each pay the rent and your half for any of the bills. Don't ever let the roommate get behind and you pick up the slack. If they cannot pay, be sure you pay your half and keep some proof (reciept or copy of check) so that if it ever goes sour, you are completely covered!