A lens with a larger maximum aperture will give a shallower depth of field, meaning the background will be blurred out more. A lens with a max. aperture of 2.8 works great.
Dealing with the sun can be an issue, a spot on the track that has good lighting at 11am will be completely different at 4pm. Move around a lot to find what sections of the track have the best lighting at that particular time.
Also take into consideration any distracting elements in the background such as port-a-potties, cars, etc. These can distract from the rider.
Composition is very important as well. Moving the focus point away from the center of the photo can be a good idea.
If you have any more questions just ask.