Living with a roommate is not easy. By creating a roommate agreement before you move in, you can prevent many problems that may come up during the next year of your life. The agreement may help roommates thoroughly discuss their needs and responsibilities prior to signing a lease together. See Roommate Agreement.
Just remember that the roommate agreement is not binding on the landlord. Any of the roommates who may be left behind after a roommate moves out will still be responsible for the full rent and all utilities until the end of the lease. To recover these expenses from your roommate you will need to sue the roommate in Small Claims Court.