Get to know each other. You've got your first opportunity to make a new friend. Take the time to ask and answer questions - about family, hobbies, academic interests, and so forth.
Talk it out. Communication is key in building a successful relationship.
Be open and friendly. You and your roommate may not have the same taste in clothing or music, but you likely share some of the same concerns and apprehensions. Be the first to step out of your comfort zone.
Be understanding. Everyone has bad days. Remember that you may not always be aware of the issues your roommate is dealing with, but there's most likely a good reason for his/her actions. A sympathetic ear might help a lot.
Give each other space. Togetherness is great, but too much of a good thing sometimes isn't. You and your roommate both need alone time. If that time doesn't pop up naturally, talk about it.
Ask before you borrow. Suddenly you have twice the wardrobe! But everyone has a different comfort level when it comes to lending belongings. Asking first helps avoid misunderstanding.
Define "neat". Whether you're a neat freak or a slob, you have someone else's feelings to consider. With a little give and take, you can each adjust accordingly and make your environment comfortable.
Discuss visitation hours. Talk about when it's okay and when it's not okay to have visitors in the room.
Pick one co-curricular activity to share. There's nothing like having something in common to care and talk about. If you aren't involved in any of the same activities, you are likely to see each other at bedtime only. You may feel like you're sleeping in a room with a stranger.
Schedule study times. Let your roommate know in advance when you have a big test or paper coming. He/she will know to give you some quiet time. A roomful of friends when you are stressed may lead to a bad scene.