I established residency for my master's degree at UCSD (coming from GA) and it wasn't difficult. The in-state tuition for UC law schools has been going up steadily, it's not as much of a steal as it used to be, but still a pretty good bargain (about $10K less per year).
You have to convince them that school is not the only reason you are in California, i.e. that you want to become a permanent resident of the state. This includes getting a CA driver's license, registering your car in CA, registering to vote in CA, paying CA state taxes, making your CA address your 'permanent address', opening bank accounts in CA, etc. It was a pain in the ass and required a lot of documentation but my department actually required me to become a CA resident since they were paying my tuition and fees so there was not much I could do. Glad now that I have it though.