When I wanted to attempt to play football in college at Pitt, which was a dream so quickly shattered, I had to work on strength and speed. As for speed, I dropped close to 4/10 of a second off of my 40 time. As posted above interval sprints work very well. I'm not sure if I trained properly because i'm pretty much new at this, but exploisive strength exercises helped a lot, like cleans, squats, clean and jerk etc.. Also work plyometrics to get flexibility and strength in supporting areas. If you've ever played football, it's all those crazy warm up drills that you did for 15 yards. All that needs to be done in the case of plyos is extend them to 40-50 yards, like majorettes (leg kick outs) high knees, fast feet, carioca, backpeddling, squatted shuffle, also if you can try to do resistance sprints, with either a parachute or weight, I found that that did lots for my endurance and speed. That's all I can say, but I did see results