It sounds like some of the best tips have already been posted.
You won't have a problem with running out of memory with 2- 128mb cards, so take your pictures at the highest quality you can, and if you wnat to save space later you can decrease the size and quality.
With the auto focus, you want to focus on the area where the rider will be before he gets there, to do this, point at an object that is about the same distance as where the rider will be, and press the shutter bottun half way, holding the button half way will keep the focus point so when the rider gets in the area your camera will be focused.
Follow the rider with the camera before snapping the picture, and follow through after the shot.
But the best tip I can give you is to practice, and try different features of the camera to see what gets the best picture. If you want to check the link for "my pictures" below, there are some shots from last years nationals, most of them were taken with an Olympus C-700.