Keep common areas tidy. Keep your personal items from taking up permanent residence in shared areas. The living room should not be a dumping ground for piles of unfolded laundry, teetering stacks of magazines, or crusty cereal bowls!
When you are done with something, put it back. Just because your roommate seems to always leave her stuff laying around doesn't mean you should, too. Especially in households with three or more roommates, messiness is a slippery slope. Set a good example, and it's more likely your roommates will follow suit.