I play corner for my high school's football team and used to play sweeper for my high school soccer team until I quit to play football (some useless info.), so I have a fair amount of speed.
In 7th grade, I was the fastest kid in my grade. But going into my sophomore season, I was one of the slower guys on my high school football team. I was starting to panic before two-a-days because I wanted to start on varsity at either corner or free safety and I needed the speed. I didn't know how I lost my speed in just two years.. until I came to the conclusion that I hadn't put enough emphasis on working out my lower body.
How often do you squat? If you don't, start doing it. That alone made me a lot quicker and since it builds muscle faster than any other exercise, you'll get results fast. If you don't go to a gym and don't have a smith machine, then just get a barbell and use that. I would also do lunges.
If you have a treadmill or go to a gym, put the slope at 8% and run anywhere from 6.5-7MPH. Not only does running 5 minutes in those conditions get you in better shape than running 40 minutes at 0 or 1% slope, but it also works out all the muscles you'll be using--especially your calves.
I have a friend that took a completely different approach and just focused purely on becoming more flexible. Whatever he did worked, he now runs a 4.3 "40.