It's difficult enough to have privacy in a house, so privacy in a 12 x 12 dorm room is next to impossible. However, it can be done if both roommates are willing to cooperate.
Tie a scarf to the outside doorknob as a symbol of Do Not Disturb, but be careful not to do this too consistently. Forewarn your roommate beforehand about possible days and times when this scarf will be on the door so your roommate will have the ability to make plans ahead of time instead of it being a surprise, especially if you plan to have an overnight guest. Be considerate if your roommate needs to get into the room regardless of the scarf on the door to get something out of the dorm because it is his dorm room too.
Use a shower curtain to separate one side of the dorm from the other. It's a little difficult to do this with a shower curtain on a wall because the pole tends to slide down when the shower curtain is repeatedly moved. It's much easier to take one side of the shower curtain or a blanket, tape it to a steady piece of furniture (i.e., desk), and then tape the other side of the blanket or shower curtain to the wall.
If one roommate is a light sleeper and the other is a heavy sleeper, the heavy sleeper could consider an alternate way to wake herself up besides an alarm. Heavy sleepers tend to ignore alarms, but sometimes vibrating cell phones feels like someone is shaking them. Using a vibrating cell phone is also a quieter way to not disturb the light sleeping roommate.
Take cell phone calls outside of the room. Your roommate may not want to hear your conversation or you may want some privacy, but your roommate should not have to leave every time you have a phone call. The roommate who is not on the phone has no way of knowing how often you will have phone calls and should not be put out because you do.
Ask if it's okay to have a guest in the room. Although your roommate is not your parent, and you may feel like this is your room too, remember that it's also his room. He may not want company when you do. If one roommate is more sociable than the other and guests become a repeated argument, consider asking the resident director if you can swap roommates with someone else.