First of all they need to know that you are there and how to get in touch with you. Having a website where you can show off your photos from a race weekend and make online sales is another thing good to have.
There isn't much money to be made at the club racing level. Most of the guys racing would rather spend their money on tires or fuel.
However, if you are able to get inside the fence and are able to get some good shots of the racing action, you can make some money selling one offs here or there especially if you catch a cool pass or something else noticeable.
You could offer a package deal where you shoot a certain rider/s for the whole season and cut them a CD each round. But this commits you to shooting every round. Still might be doable.
You can make more money shooting car and bike trackdays. A good thing to do is partner up with multiple trackday organizers. In a lot of cases they want you there as a "value add" to the various services they offer aside from just the trackday, like tires, video, suspension services, instructors, etc.
Depending on your agreements, you may or may not get exclusive rights to shooting an event.
Like you've mentioned before, some mobile outfits have some pretty good setups with a dedicated trailer with multiple viewing stations and an assistant who will sort the days photos, cut CD's, make one off prints, while the photographer is out shooting.
This is probably the only way you would be able to send customers home with their photos on the same day of the event.
Without the trailer setup and assistant, you are forced to shoot the day, sort the photos when you get back to your hotel/home, and then upload them to your website. Then hope that people contact you for their photos. You would then cut a CD and mail it out. You lose the impulse buys.
You can start out small. A 12'x12' canopy, a generator, a laptop for sorting and burn CD's, and maybe 1-2 large screen laptops as viewing stations.
Pass out business cards during rider/driver down time between sessions.
Whether you decide to shoot club racing or trackdays or both, make sure you get permission from the organizer as well as permission from the track. Develop a good relationship with everyone from the organizer, to the track officials, to the corner workers.
Hope this helps you get started and good luck.