When it comes to roommates, communication is key. The willingness to talk before problems arise and to confront the problems that do occur, are critical to successful relationships. "Successful" doesn't have to mean you've become best friends. It means you're mutually civil and, most importantly, you're both able to use and enjoy your own living space by cooperating and being considerate.
Nothing in common
Don't be surprised if your roommate is nothing like what you expected — whether you're rooming with your best friend from high school or a foreign exchange student you never met before. Living with someone is very different from going to class or hanging out with them. You won't know how it'll go until you've both had opportunities to show how much you're willing to make it work.
Remember that your differences can be a good thing by allowing you to share new experiences with the other person. If roommates make a strong connection, that's great, but the priority is to be cordial, civil, and respectful.
Knowing when to speak up is important, but it's also vital to know when to keep quiet. These skills help you smooth over rough patches before problems escalate into tense situations. The most important step is how you present the situation. Be direct but use a light, nonconfrontational tone. An overly serious, dramatic, or, worst of all, angry request won't get results. And a touch of humor can only improve your chances for success. Be careful not to be too casual with a request — the roommate may not take you seriously.
Lack of privacy
Living in close quarters means a lack of privacy. Gaining privacy can be as simple as closing your door (and maybe locking it) or putting up a "Do Not Disturb" sign. If you live in small, open rooms with a roommate, learn your roommate's schedule and take advantage of the time that she'll be at class, work, or activities. And then there will be days when you want to be alone but your roommate is likely going to be in the room all day. In this case, you'll have to find privacy somewhere else. Go to the library or student union. Go for a run or to the gym. There are plenty of ways to find solitude.
Roommate from hell
If your roommate snores sometimes, wakes you in the middle of the night on occasion, and is habitually messy, congratulations — you have a typical college roommate and will likely receive little sympathy if you request a room change.
You might have a case, however, if the roomie
Regularly disrupts your ability to sleep or study at all hours of the day, and refuses to make changes
Engages in illegal activities
Smokes (and you don't)
Endangers you physically in any way
Interferes with your practice of religion
If it's impossible for the two of you to get over your conflicts, do your best to persevere and try not to make a bad situation worse. Don't be equally obnoxious just out of spite. You can put your headphones on or go hang out elsewhere.
This isn't to suggest that you should avoid your room all the time. You live there and should feel free to come and go as you please. If you're stuck with a roommate you really can't stand with no chance of resolution, it's healthier for you to minimize the time you spend together. Even if the two of you loathe each other, you both might agree to scheduling time to be in and out of the room.