Training for 500 Free

Hello, all: I just recently joined these forums but was hoping I could get some tips on how to improve my 500 free. My current times lie around 5:40 (PR is 5:35), usually with my split from a dive being around 28.00 then having the rest of my splits level off to around 34.00-35.00. My goal is to have my time down to a 5:25 by memorial day weekend (15 weeks), and eventually down to under 5:00, although I haven't set a time for that. I currently swim 3800-4200 5 days a week with no real focus in my workouts. I plan to up my workouts gradually so that I'm swimming around 30,000 yards a week (up from 20,000). What things should I be focusing on in my workouts to help my splits? Are there any specific aspects of swimming that I should be focusing on for the 500? Thanks, bow