Put It All on the Table
When I say you and your roommate should put it all out on the table, I don't mean you should walk into the room and dump your stuff out. As you get to know your roommate, start talking about how you two envision sharing your room. When do you expect to go to sleep and get up? How neat do you like things? How many hours a day do you plan on studying in the room? How often do you like to have people over? Are you okay with sharing things like clothes, food and toothbrushes? And yes, the toothbrush sharing thing happened to a girl in my freshman hall. If only she had thought to tell her roommate that she didn't want to share her toothbrush.
A lot of colleges require roommates to write and sign a roommate contract. Even if your school doesn't, you and your roommate should still do it. In the contract, spell out how often you plan on cleaning the room (and who's going to do it), what the room's quiet hours will be, how you two will handle visitors (how many, how often and can they stay overnight?), if you're cool with sharing things and generally anything else you can think of that could possibly lead to conflict.
You and your roommate will probably have to compromise on a lot of this. That means you'll both have to make concessions — don't employ whatever mind tricks you know just to get your way. If you want lights out at 10 p.m. and your roommate wants 2 a.m., split the difference. If your roommate is a clean freak, you should agree to pick up after yourself; but she shouldn't expect you to attend her Friday night dust-a-thons.