If you're a neat person, living with a messy roommate can be impossible. Many people choose to end their roommate relationship over differences of opinion on how neat an apartment should be kept. However, if you can't end your relationship, or would prefer not to, About.com's tips may help save your relationship and your sanity.
First off, don't jump to the conclusion that your roommate is keeping things messy just to make you mad. There could be lots of reasons behind your roommate's behavior. Perhaps they have a different standard for what constitutes clean, or maybe it just doesn't bother them at all. On the other hand, your roommate could have lost their motivation due to personal problems, such as a breakup or being unemployed. Another option is that your roommate would like things to be as spotless as you do, but she doesn't think she can do it. Sit down with your roommate and discover the roots of why they are like they are. Then you can work on a solution.
Look at your apartment as being broken up into personal and private areas. Your room is of course your zone to keep as neat as you'd like, and conversely, your roommates room is theirs to keep as disorded as they want. Don't bother discussing these private areas, focus on standards for the bathrooms, kitchen and living room.
Propose hiring a professional cleaning service and splitting the cost. That way the apartment can stay tidy without anyone having to do so much work. Plus, the price will be lower because you can split it. As a bonus, your roommate may be motivated to keep the house neater between cleanings.
If it's easier for you to keep the place clean than your roommate, you could suggest doing more cleaning of the shared spaces in trade for your roommate doing more of the other chores. Your roommate could do the bill and rent paying or go to the grocery store or any other shared chore that needs to be done.