Before you start, have a look at the tips from Danil:
Action is awesome but have a look around, some emotions from behind the scenes can also make powerful images.
Be careful with taking wide-angle pictures close to the rider. You may get bike into your camera.
Take a good care of your gear. You don’t want it to end up damaged or stolen.
Talk to the riders you shoot. Ask for their opinion about your shots. I bet they have some ideas about the photos too.
Be creative. Try long exposure to show the dynamics of BMX sport or double exposure to mix portraits and action scenes for example, or other creative and unusual methods.