Lacking more telephoto, you can also do the old-fashioned thing: get closer. Are you shooting from the stands, or down on the sideline of the field? If they allow it, and you are not already doing it, get down there along the side of the field (on the side opposite the coaches' box and the sideline referees), and move with the action. When they go to the end of the field toward the goal, you move accordingly.
Definitely use spot focusing, and pre-focus (shutter button part way down), following the action. I use "burst" shooting mode, too, as I follow the action. I may shoot a dozen or more rapid-fire shots at a time, then later cull down to one or two good ones. For a typical youth soccer game, I usually shoot over 400 images. Hey, film for digital cameras is cheap!!!
