1. Set Expectations Early On
As early as possible, make a point to sit down and discuss expectations. Discussing your expections about cleanliness, shared belongings, visitors, etc. can prevent misunderstandings down the line.
2. Make a Point to Get to Know One Another
Try to schedule a monthly roommate hangout or dinner. The more you know your roommates, the more comfortable you will be communicating with one another.
3. Address Problems in the Moment
If your roommate is doing/saying something that bothers you, speak up quickly and respectfully. Waiting to address the issue or avoiding the issue entirely can build animosity. All that built-up tension will make it more difficult to keep your cool.