Don't try to get sponsored if you aren't interested in putting in some work. Otherwise you're just a drain on the industry. Be prepared to put some time in at demos, comps, and generally pumping up you product.
That being said, the best way to go about it is make a vid showing your best stuff. Also, write a resume. Be sure to include how you can help your sponsors out. They hear from thousands of kids wanting free stuff. They're only going to help you if you can help them. From there, try to meet up with reps or contact some of the smaller companies that have a good grass roots program.
Sponsorship is as much about attitude as it is about riding ability. I have a couple friends who are sponsored, not because they are the best riders around, but because they are good with people and at advertising their product. Be confident and friendly with people, and don't be a jerk. That will get you a long way.