Well getting sponsored wouldn't be an easy thing, you gotta go to events. Keep a clean car. If there is a praticular company you want to get sponsored by, you should buy most of their stuff. Just keep your head up, write companies letters, and just wait untill the day comes.
Start by e-mailing the companys them self. Be sure to send them pix of your car and a list of your mods. OR go in to any of your local speed shops and ask them for a sponsorship. They will probably be easier to deal with, because of the fact that they will have a wider selction of items and you wont have to be sure that you have all of one company.
get lucky with one of the ppl who own the company j/k...just know ppl or do what everyone else said....or if u want a shop to sponsor u talk to the boss and ask if u can get some minor work done for free and u'll stick his shit all over u'r car (so long as it aint tacky) and see where he goes from there with the sponsorship...
go out, spend thousands of dollars on your car. start with like 5 grand or so, then sign up at a buncha sites (many aftermarket sites offer sponsors) with there parts listed on your car and the several car shows you frequent each year.
being sponsored isn't just putting a sticker on your car...believe me