Plyos are good. Ever try bungee assisted sprinting?? Do it both pulling someone else and being pulled. I'm no doctor, but i know it forces your muscles to become faster. I did this when I used to play football, I was 6'4 285 and knocked my forty from 5.2 to 4.8 in high school, that was the main reason i ended up getting a full ride to school. I swear by this for speed.
-K