If you have concerns about certain issues that may arise, such as nighttime noise levels, overnight guests, cleanliness, drug and alcohol use, etc., discuss them with your roommate early on. A friendly, open conversation at the beginning of the year about your needs and preferences, and those of your roommate, can help prevent problems in the future.
This is one of the most basic and most frequently overlooked steps in fostering a healthy living situation. Many people are uncomfortable with confrontation and express themselves instead in indirect, often passive-aggressive ways. The fact is that people are not mind-readers and passive-aggressive tactics tend to lead to increased resentment in both parties without really fixing the problem. Be open about things that bother you. If your roommate is doing something that disturbs you, express it in a way that is polite and non-accusatory. Your roommate is much more likely to respect you and your wishes if you address matters directly rather than dropping hints.
Don't let silly things color your perception of your roommate. You may not like your roommate's taste in music or choice of decorations, but that doesn't mean you can't get along. In fact, you don't have to love or even like your roommate to create a healthy living situation, as long as both parties are respectful of one another. Moreover, you can't control what your roommate does outside the room, and you shouldn't try. Confine your roommate concerns to things that directly affect your well-being and let the other things slide.
Sometimes roommate conflicts require third-party mediation. If you feel a conflict is beyond your control, discuss the matter with your resident adviser (RA) or your dorm's equivalent thereof. Concerns about your roommate's safety or well-being as related to stress, anxiety, depression, health problems or drug and alcohol abuse should be addressed to your RA as well.
Some people just shouldn't live together. If you reach a point where attempts at conflict-resolution have failed and your roommate's behavior is significantly disrupting your sense of well-being or your academic performance, the best course of action is to change rooms. Discuss your options with your campus' housing office. College is stressful enough; you don't need to let yourself stay confined to a bad living situation.
Always show your roommate the same respect and consideration you want to be shown. It may seem obvious, but it's surprisingly easy to forget. The best way to get respect is to give it.