Communicate openly with your roommate. Agree to talk about issues before they get out of control.
Accept that everyone gets in a bad mood now and then. Give him plenty of space.
Decide early how the rent and utilities will be split and what to do if one person can't make the month's payments.
Establish rules about food, company, chores, noise, tobacco and alcohol use, and parking. Make sure each person's expectations and responsibilities are clear.
Respect your roommate's right to privacy and his possessions.
Always ask before you borrow anything.
Knock before entering his room.
Don't tie up the only phone line ' consider getting separate ones for each of you.
Begin your statements with "I," as in "I get really annoyed when you leave your shoes on the couch" instead of "You're a complete slob!" Focus on his behavior, not him personally.
Recognize that some people are neater, louder, more private, or more possessive than others. Don't expect your roommate to mirror you.
Treat him the way you expect to be treated.
Ask for what you want. Subtle hints are likely to be missed.