During the off season, work on a full body weight routine. But, you want to build strength without sacrificing endurance and aerobic fitness. Work on developing both upper and lower body strength. During the season, since there is a high amount of stress put on your lower body, I recommend weight training for your upper body only. Since your legs are worked out hard on almost a daily basis, training them with weights is almost too much during the season. It's important to have good upper body strength because as you get older (college and the pros), the game becomes more physical and you need upper body strength to hold off defenders, keep good balance and protect yourself in the air. College and pro soccer players are some of the most physically fit athletes you will find. Muscle and strength development is very important.