the best advice I can give is...

- make a portfolio. Get some pro quality pics, list the mods, get pics of the build, list future mods... Put together a nice portfolio in both printed and digital form to give to potential sponsors. Put your contact info in, too.
- along with above, keep a few personal info cards in your car and on you. That way it you're randomly at a bar or something and the guy drinking next to you happens to be the owner of a tire shop, you can give out a card instead of scribbling down your number.
- give your info to as many people as possible. Even if its a long shot, give your info out anyways.
- show, show, show.
- show up at drag race/autox/drift events. I stay the hell away from street races, but try to appear everywhere else.
- just talk to people. Most companies will give you discounts in exchange for you sporting their vinyl. Thats a start. Eventually you can say "hey ive been advertising for you for a while and have spent a lot of money with you... How about returning the favor?"