in graduate school, at least in PT school, your automatically considered independent of your parents. on the FAFSA it will ask if you want to provide information about your parents finances, and you don't have too since you are considered an independent. So filling our your FAFSA is a lot easier in grad school, that is to say if you don't have a full-time job, investments, real-estate, etc.
Your parents can still claim you on their tax returns without a problem. However if you are an out of state resident, some states actually allow you to establish residency while going to school full time, the state of washington is one such example. As one of the stipulations to establish yourself as a resident, your parents cannot claim you as a dependent.
A lot of people get worked up about having that "independent status", but ultimately what it boils down to is that the government will allow you to borrow more money, both subsidized and unsubsidized. In grad school, you can borrow $8500 subsidized by the government, and an additional $12500 unsubsidized.