Hi gally,
You've gotten some good advice which I would heed. Anyway, here's my two cents...

My recommendation is to implement the Tabata protocol into your sprint training regimen. This is a protocol established by Dr Tabata of Japan when he was tasked with finding the optimal way of most effectively and efficiently developing the anaeorbic capacities of the Japanese Olympic speed skating team members. What his research bore out is widely considered, in recent years, to be THE "magic bullet" of the sports conditioning world in terms of interval training. Okay, enough background; here's the protocol.

20 seconds of anaerobic work (at the highest intensity you can muster for that duration) followed by 10 seconds of complete rest repeated eight times. Total time: four minutes

Any exercise, which can be continuously sustained at a high intensity for 20 seconds, may be used.

So, among other exercises, you can, for example, use sprinting (on flat lands or hills), hitting the heavy bag/punch mitts (my fav), rope jumping, power cleans, squats (try overhead squats for a killer workout), burpees, etc. I think I've painted the picture. Anyway, give it a try with the exercise of your choice. I think you'll love this protocol. I use various interval training schemes for variety, but this one is among my favorites.

If you want fiurther information on this protocol, do a google search to learn more about Dr Tabata's research. You'll be impressed because not only does it improve your anaerobic capacity significantly but also your aerobic capacity at the same time. According to his (Dr Tabata's) research, any variance from this "magical" 20 sec work/10 sec rest/8 rep scheme nullifies this dual benefit.

Hope this helps you out...

Take Care,
Lito