I'm basing all of this on my amazing roommate, Emma, who is quite possibly the best roommate and friend anyone could ever have. Basically, just be her, and you'll never be able to go wrong with the room-sharing situation.
1. Introduce yourselves BEFORE you get to campus
Make sure you shoot your roommate an email before you both get to campus. Introduce yourself, let them know who you are, what you like, things you could potentially bond over, things that you need them to know about yourself, exchange phone numbers. Be friendly and welcoming, but also clear. Even if all you really want to do is figure out what shared items you're each bringing so you don't bring duplicate stuff, make sure you reach out. It can be really comforting knowing that the potential first person you'll meet on campus is friendly.
2. Wish each other a good day
Emma always wishes me a great day, no matter what's happening. Whenever I'm leaving out the door, she tells me to have a great day, to have fun at my event, to feel better if I was having a bad day. If I've already left, she shoots me a quick text full of smilies and happiness. And honestly, it makes all the difference. Just knowing that someone out there cares about you, is thinking of you, hoping you have a good day puts such a positive spin on the day. It truly makes you feel better. Always make sure you do this, with your roommate, with your best friend. It's important to know someone is out there.
3. Keep up with each other's schedules
Especially with the important things in their lives. This goes hand-in-hand with wishing each other a good day; you should know when the other has something big coming up, and offer support in anyway you can. Even if that's just shooting them a text to wish them luck or tell them they've got this. Knowing the other person's schedule also means that you'll be respectful of their space. If they've got a big midterm the next day, you'll know not to come in at 3 am being extremely obnoxious. Keeping up with schedules is really important when learning to respect each other's space.
4. Ask if you're going to use each other's stuff
Even if we've agreed to share everything, if something's on her side or that she specifically brought for herself, I always try to ask before I use something of hers, and she does the same. It's the simple matter of respecting each other's space. This is really hard to do sometimes, especially when you've never lived with someone before. But just making sure you pay attention to their feelings and what they say, and when you're communicating make sure you're always clear and succinct. It only makes everything work much more smoothly.
5. Be clean
No matter what, people just don't like living in a messy environment. Keeping your stuff tidy and kept together, cleaning up the floors, your dirty laundry, etc., keeps your roommate happy. Emma and I pride ourselves in our meticulously kept room. Neither of us has ever lost our stuff or tripped over each other's stuff while in our room, while so many of our friends have. It's just so much easier when everything's clean.
6. Talk about the things that are bothering you
Never keep anything that's bothering you inside. Never ignore the problems. Always talk about them. Always talk about the issues you're facing with each other. Communicating works. It makes the problems easier to solve. They won't know what's bothering you if you don't tell them. Roommates are regular people, not mind-readers.
7. Learn each other's quirks and do random acts of kindness for each other
If you know your roommate likes the cookies from the dining hall, maybe you bring them back a cookie after you go to lunch and leave it on their desk. Maybe they forgot to do something, and you help them out. Maybe you leave them a cute note. Whatever it is, little thoughtful acts keep the peace between you two and make the other person's day.