As a soccer player myself, I've learned that the core is the most crucial part of the human anatomy for the sport. As you know, soccer involves plenty of running. In training, not only do we workout our lower body (quads, calves, etc) but our core. You have to keep in mind that leg power is derived from the core. Look up ab exercises, as well as core. Try parachute training, and study TECHNIQUE. A good acceleration and takeoff can be all the difference in a race. To help with this try Plyometrics. These exercises increase flexibility and explosive agility. With 1 month of dedicated core, leg, and plyometric workouts supplemented with diet and you will notice significant change.