Agree on an acceptable level of cleanliness. Many arguments between roommates stem from different ideas about cleanliness. If one of you is constantly cleaning up after the other there is bound to be conflict. Work out a chore schedule as soon as you move in together and stick to it.
Be considerate. When it comes things like noise, parties, study time or bringing dates into your shared living space always be considerate of your roommate. Have an honest conversation about sleep and study patterns, acceptable noise levels and overnight guests. Be willing to compromise. Respect your roommate's needs. Set clear expectations upfront about what you expect from one another so that no one ends up frustrated.
Communicate to resolve conflict. Remember that you and your roommate are not friends, and living with someone you do not know requires communication. Your roommate can't read your mind, so it's up to you to let her know when there is a problem. Be honest about your needs and do your best to accommodate hers. Talk about issues as soon as they come up, or your roommate may not even know there is a problem and the issue may continue.
Respect your roommate's space, privacy and personal items. You know those days when you just don't feel like talking? Your roommate has them too. Allow your roommate to have his space. Don't go rummaging through his things and stay out of his room unless he gives you permission to be there.