The usual sports photography basics apply to camera settings, set it to aperture priority and use the lowest f number available. If that's giving you a lower shutter speed than 1/1000th (800th minimum) then up the iso so you can freeze the action. Use focus on continuous mode (is it called servo on Canons?) with just the centre spot selected. Aim to get focus on your target through the centre spot before firing.
After that, less of the normal sports photography rules apply to American Football. Shooting from a low viewpoint always makes for better pictures, espcially as you want to see players faces under their helmets. Scott is talking sense however, there's always the danger that the action will spill over the sidelines, and you really don't want to find yourself under it! I've had one or two close shaves over the years and nowadays I always stay on my feet when on the sidelines. Once the play gets within 30 yards of the goal line I sit behind the in-goal area to get the action coming towards me.
There are so many different things going on during a game that the usual 'make sure you get the ball in the shot' doesn't always apply. Many of the players will rarely even get close to the ball, but they still like photos showing what they're doing.
The basic shots to get are of a running back coming towards you, or of the quarter back passing or handing the ball off. I vary things by working ahead and behind of the line of scrimmage to get different views. One of the good features of the game is that there's time to move position between plays. There's lots of blocking, tackling, even kicking occasionaly, going on to get interesting photos. Try getting close ups of the linemen battling as the ball is snapped.
I'm really looking forward to my game on Sunday. Enjoy yourself.
