Dave, are you standing up when taking these? Try and get down as low as possible so that your camera is below the level of the players faces. Small camping seat is ideal.
Unless theres something in the background or foreground that adds to the action, crop tight to the players and the ball.
For headed shots, try and predict which players are going to head the ball and fire a few shots before, during and after the heading takes place. It gives the camera time to focus and you'll get one or two where the ball is within a couple of feet of the players heads or if you're really lucky right on the players head. Remember it ain't film, it doesnt cost and you might just hit that killer shot.
For action shots try and hold the picture until two or more players are involved in the play. ie player with ball and opponent tackling.
Above all enjoy it. Its a real challenge and great fun.
See below.