Having a roommate contract and a clear understanding of what each of you expect from this situation helps when your roommate does something that royally ticks you off. And believe me, that will happen. When it does, speak up. My relationship with my first roommate was bad, in part, because neither one of us ever told the other when she was doing something aggravating. So I never asked her to take out the trash. I just seethed about always having to do it myself, until one day I snapped and put the trash on her bed. Not my proudest moment (okay, it sort of was).
If your roommate is doing something that you don't like or that violates your contract, speak up. Don't let it fester. They key, however, is speaking up in a productive way. When your roommate staggers in at 4 a.m., that's not the best time to leap out of bed and start screaming that he or she stays out to late and always wakes you up.
Wait until you're calm and collected to talk to your roommate. Use clear statements like, "When you play the trombone at two in the morning, it wakes me up and then I have trouble concentrating the next day. Can we work on deciding dedicated hours for you to practice? Or maybe you could use one of the practice rooms in the music building?" Try to avoid making accusations like, "You never do any laundry and it makes the room smell like crap!" That will just start an argument. Instead, state how a given behavior makes you feel, and look for a way to solve it -- something that works for you both.
Or, you can just put the trash on his or her bed. Your call.