Find a park with a gradual decline to it (not more than 15 degrees), Mark your sprinting distance and sprint 20 times up it. On the 20th trip up you stop at the other end. Stretch your legs and run 10 sprints down hill. This will both build your muscles in your legs and lengthen them to teach your legs how to turn over faster so you do not fall on your face. It go me from a 5.5 sec, 40 yard dash to a 4.85 sec, 40 in one summer back in the day when I played football.