I would read, digest, understand, then reread, digest, understand the Maxing out @300 thread (stickied in technique). It is overwhelming how much good information is in those 30 or so pages. I want to say somewhere between pages 4 and 15 is a good nugget of information on hip rotation/use of core.
I'm right around the same distances that you are, I get about 280-300 with most fairway drivers, stretching out to~330 on a great throw with a Valk or an OLS. I've been working Bradley Walker's closed shoulder snap drill my last few trips out to the field and really like the results. Much more consistency already and for whatever reason I have a much easier time with his drill rather than the right pec drill in keeping the nose of the disc down. I get the results I think I should be getting from the pec drill (fast straight line drive throws) with the closed shoulder snap drills. You can find the video on youtube under citysmasher1.