Some advice and reaffirming some of what has been said above.
Shutter speed: try to keep above 1/1000 to 'freeze' the action, you can go slower but you run the risk of blur effects on the legs etc.
Aperture: go as wide as your lens will allow
ISO: I would set to auto 400-800, depending on the max aperture of your lens and the conditions, you could always go higher if you're not getting the shutter speed
Position: obviously you want to photograph your girl, so attacking end for her team, 5yds in from touch, get yourself a travel stool and a monopod, if your lens has a tripod mount, you'll find panning a lot easier
Back button focus: yes, always - it's second nature once you get used to it
Skill: look for the bursts of action, once you see it, it'll become second nature, tackles, drives, rucks etc. niche - the scrum-half looking over the pack is a classic, but you'll need to move position (which isn't a bad thing), personally I wouldn't bother with moves going away from you, no one needs pictures of people's backs.
Drive: continuous high, the action is often so fact you'll need that extra shot for open eyes/hidden face etc.