The secret to speed training is to apply power to good running mechanics. Knowing the proper sprinting technique is crucial.
Maximum velocity isn't achieved until 30-35 meters into your run. So for a sport like football where you are more interested in shorter sprints, maximum speed may not be the number one goal of your speed training.
Acceleration becomes the desired outcome of your training. However, training to improve maximum speed also will improve your acceleration, power, coordination, and dynamic flexibility.
Posture
Because your head is heavy it tends to go wherever your eyes look. Since you want your head to stay upright you need to concentrate on focusing your eyes on the horizon.
Keep your abdominal muscles tight and your hips in line with your torso. The muscles of the abdomen stabilize the pelvis in the optimal position so that hamstring and gluteal muscles can apply maximum leg force against the ground. They also allow the hip flexor of your other leg to recover in time for the next stride.
Shoulders need to be slightly ahead of your hips during a sprint
(relaxed and not hunched).
Pelvis should be forward and upward creating a tall posture.
Arm Movement
Incorrect arm action is a mistake that many players are unaware that they have. The arms are important to counterbalance the motion of sprinting.
Keep your elbows at right angles (that is 90 degrees) and move them forward and back, not across your body. Drive your arms back forcefully by rotating the shoulders. Your hands should go from eye level to hip level.
The action of your right arm effects the left leg and vice versa, therefore the arm action must be powerful to counteract the hip rotation caused by leg movement.
If not holding a football the hands should be relaxed. As soon as your hand returns to shoulder height it should be immediately swung backward again.
Leg Movement
Don't stretch to achieve a greater stride length. The leg action should occur under the body. Plant your foot under your hip or as close as possible. Over-striding can actually slow down a runner.
After you push off with one leg your ankle should be dorsiflexed (toes up toward your shin). Your heel should come close to you butt. This results is faster leg turnover.
As your recovery leg comes forward your thigh will reach a point where it is parallel to the ground. Now your leg unfolds at the knee and hip extension begins due to momentum. Don't use your quadriceps to reach further, or you will begin to over-stride which can actaully slow you down.
Now your leg makes explosive ground contact under your hip, with your ankle being being used as a springboard. Elastic force production is essential. Try to have the shortest ground contact time possible.