As an expat Brit myself who has been to several slug-fests, erm... Rugby League matches, is to think of these games as American Football without the protective uniforms.
Assuming you have the basics covered - are on the sidelines so to get shots of the action, you'll have a lens that goes out to at least 300mm and is as fast as you can get. I use a D7000 body as it works well at low light... Shoot at the fastest shutter and ISO speed you can get to without too much quality loss.
Look for action as the ball starts to move and when players move together, collide, tackle (try to isolate them from the rest of the action and you'll get some great shots).
Nobody will think any less of you if you use a monopod if the game is at night or it's an evening game as the sun goes down. Quite the opposite in fact - it's not a street photography walk about.
You can try and get the best CLOSE UP shots too. I have seen some really good images of faces with contorted expressions at these games. Moderate length (135mm) lens and if it's been raining and there is mud, so much the better. Lots of visual interest when it gets a little bit messy.
Same kind of advice as for Football... Be in front of the line of action as things move up the field. Observe how photographers who do this all the time place themselves and very important... If players are about to hit you/run all over you, drop your gear out of the way and lay down... *THEY* (the players) are the reason for being there, not you... *THEY* will usually not stop or run round you *THEY* may try to jump over you, but *WILL* collide with you if you are standing up!
If you enjoy it, buy a place on a sports shooters seminar and see how the pros do it. LOTS of tips about placement for getting good results at the better of those teaching events.