Don't play into your housemate's passive aggressive games. If she leaves you hostile little notes, don't write notes back. Either ignore the notes entirely or confront your housemate directly. The same goes with housemates who use your stuff, don't keep up their end of house chores, or talk trash.

Keep things calm. Shouting matches with people you love are rarely desirable. Shouting matches with people you don't like very much are even less so. If you can't talk calmly together, take some time to cool off.

Get out. Spend more time with outside friends doing things outside the house. Stay over at your boyfriend or girlfriend's house. This will help you to cool down, and may diffuse the situation as well.

Make sure to have your own space. You can live with someone you don't like provided you have a room to retreat to. Make sure that it is as comfortable as possible, and that your privacy is respected.

Get a mediator. If you can find a neutral housemate, get them to mediate the dispute. A good mediator will make sure that everyone has a chance to say their piece, and that each party listens to what the other is saying.

Come up with rules you can both live with. Once you both understand that you have to live together, you can come up with a way to do it. Divide chores and come up with a way to stave off arguments.

Don't be afraid to call the cops. If your housemate is hysterical, destructive, threatening, or violent, this is always your best bet. Even borderline psychotic housemates generally calm down when the cops come.

Laugh at him. Granted, this is not the most diplomatic way to deal with, but it is a great strategy when things have gotten pretty bad and you want to feel better at your opponent's expense. If someone comes at you with a lot of anger and you respond by laughing in his face, you win. You don't even have to say anything nasty.