How can I increase my sprinting speed?

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.

Have you tried visualization, actually seeing and feeling yourself sprint faster? Lindsey Vonn, the Olympic skier credits visualization with her success.

look up fartlek training (swedish term for speed play) it is basically varying your pace (speed) from slow to fast to slow to fast. (10 minute warmup; 1 min fast; 2 min slow; 2 minutes fast; 1 minute off and repeat 3-4 times. It works on endurance as well as VO2 capacity. Strength training is always good. Dedicated Dynamic warmups avoid injuries and dedicated cool down w/ stretching afterwards