Whether you are rooming with your best friend or went potluck, living in a small space with another person can be quite challenging. I personally roomed with my best friend from high school, and we had more than our fair share of arguments throughout the year. So, here is some advice from personal experience:
1) Bring headphones. If you don't want to listen to your roommate whine to their mother on the phone for the thousandth time, pop in some headphones and drown him/her out with some upbeat songs.
2) Let the little things go. If your biggest problem is that it annoys you that your roommate has to brush her hair exactly 5 times before sleeping, you're living the good life. Unless something your roommate is doing is disturbing your sleep or study time, is against your ethics, is illegal, or is just plain rude, don't mention it. You have bigger things to worry about.
3) If they are doing something that disturbs sleep or study time, is against your ethics, or is rude, tell them right away. It's better to hurt your roommate's feelings a little bit now than to have an angry outburst later. Do this tactfully so it's not as awkward. Try phrasing things as a question. Example: YES - "Hey Herman, could you possibly skype your girlfriend earlier in the day? When you skype her at 5am it keeps me awake and I'm tired for class the next day." NO - "You're annoying when you skype your girlfriend at 5am."
4) Get some alone time. My roommate and I did everything together freshman year. We joined the same clubs, took some of the same classes, went to the C-store together, made the same new friends, etc. Don't do this. There is only so much you can be around a person before snapping. If you want to hang out with your roommate the majority of the time, that's fine. Just make sure you take an hour or two a day to be by yourself or with other friends. Good places for alone time: The stacks in the library, the arboretum, and the lounge in your dorm.
5) If you borrow something, ask first, and then put it back exactly where you got it.
6) Don't wake your roommate up. Only exceptions to this rule: a) Your alarm goes off before your roommate's because you have class earlier and need to get up. b) There is a fire. c) You think they might be dead. d) Ryan Gosling (and you must be 100% sure it is him) is standing at your door. That's it. This means don't stand in your room and call your boyfriend or your mom at 8am. Don't slam the room door when you walk out to use the bathroom. Don't scream wildy at 6am because you think a bug is crawling on you.
7) Keep it clean. Okay, not clean like your mom is stopping by for a visit, but clean enough to not be disgusting. Keep your dirty socks and underwear off the floor and in your closet. Shower regularly. Try deodorant, it does wonders. Don't put your comic book collection all over your roommate's bed. If your desk is messy, fine. If the common space (such as the floor) is, clean it.
8) Establish rules early. Decide if boys/girls are allowed over and for how long. Your roommate probably doesn't want to watch you and your girlfriend tongue each other, and your roommate also does not want to sit outside the door until 4am when you are done.
9) Be nice. Don't be a Debbie downer. If you're constantly negative and insulting, your roommate will not like you. Why? Because negative and insulting = No fun. Don't tell your roommate you think his girlfriend is ugly. Don't tell your roommate she will probably fail her test because she is dumb. This is common courtesy, folks. If it sounds like an angry kindergartener would say it, you shouldn't. Try giving your roommate a compliment a day, and you should be friends in no time, or at least tolerate each other enough not to want to kill each other.
10) If all else fails, live together but apart. If your roommate is seriously the most awful person on the planet and talking to him/her several times hasn't helped, just do your own thing. My friend that was a freshman last year had a roommate that constantly had boys spend the night unannounced, would come in at 4am drunk and puking everywhere, would ask for my friend to buy her food all the time claiming she was starving, and would get in loud arguments with her dad over the phone while in the room. That sucks (and thankfully most people are not like that, so don't worry!). After several talks failed to bring about a change in her, my friend survived it by just ignoring her roommate. She would do her homework quietly at her desk or in a friend's room. She would be civil to her roommate, but never hang out with her or have long, personal conversations with her. By doing this, she managed to make the most out of her situation and enjoyed her freshman year. She is even living in a dorm again next year! It just goes to show that no matter how bad it gets, you can always semi-get along.