Some very good advice from lots of people here as always. If the light is bad look for times when there is not much movement in the game, for instance when the line out jumpers reach their height when going for the ball, there will be a split second when their is little motion, also the camera will be metering for part of the sky which might help. here is an example taken last winter:
If your camera has an Auto ISO function use that with your aperture at its widest setting, a minmum S/S of 1/250th and let the ISO change as required.
Hope this helps.