Hello mate,
I started as the team photographer for my daughters teams last year, so I know what you're going through. First bit of advice would be to get as low as you can, I usually sit on my backside, usually on the goal line around halfway between the goal & the corner flag or on the sideline between the centre line & the corner flag. Sitting like this it also makes the camera very steady as you can lean on your knees & become your own tripod (helps if you've got a heavy lens on)
Getting nice & low also reduces the risk of chopping players heads or feet off as you'll be shooting parallel to the ground instead of diagonally downwards, it also helps to get theball in the shot.
Here's one of mine..............
IMG_4031 by
Domski2008, on Flickr