Step 1: Take a year off. Make sure you work and rent (own a home if your parents are upper middle class) in the state you wish to enroll for the four year university you'll attend. This will allow in state tuition for public universities; however, this is not needed for private universities.
Step 2: Marry, so your parent's income does not count for Effective Family Contributions (EFC). Marriage or 24 years old (by July 1) are the two easiest way to gain independence from your parents' EFC, and marriage could occur earlier.