i have a few different approaches.
one is called guerrilla/killer cardio.
for 1-4 minutes (depending on your fitness) you jog at about 50% of your max speed.
then continually for 4 minutes you sprint for 20 seconds, and rest for 10, continually.
then again 1-4 minutes (depending on whether you can walk or not

) you will walk quickly or jog and be done.
total time frame is 12 minutes...of fat burning hell.
another approach would be to measure out distances (i go to the track and do the 100m) and then i sprint it, then walk back at a slower pace and when i get the the beginning i spring it again, do it about 10 times and you're all set there.
remember too that sprinting should be treated kind of like a workout. eat about an hour before, and soon after... and i go pro/fat after, because fat burning will still be occurring for a short amount of time after sprinting.