You're right, the rule is "typically" 6-8 eight ounce glasses per day, which is 2 liters. I've read that you should add an additional glass for every extra 25 pounds you're overweight.
The other way I've heard, which seems to be the 'new and improved' way is to drink half your body weight in ounces everyday. So, if you weight 150, drink 75 ounces... if you weigh 200, drink 100... kwim?
Click here for an article about water that explains why you need it, I find it very motivating!