You'll get along better with a roommate who shares similar standards of cleanliness. You don't want to be labeled a slob or constantly prod your sloppy roommate to clean up her mess.
You don't need to have the same interests as your roommate; differences will keep life interesting and ensure you're not always together. However, your personalities should mesh well, so ideally you'll choose someone you're already good friends with.
It's wise to choose a roommate in the same year of school as you, so you'll potentially stay roommates, and good friends, throughout college.
Choose someone with similar study habits and schedule preferences, too, as eCampus Tours says. If you take your studies seriously and study most evenings, you'll want a roommate who also studies and doesn't hold wild parties on a regular basis.
Make sure you and your roommate both want to spend a similar amount on rent and that you can both make payments each month. Discuss this practicality before you start searching for a place to live.