Hey mate.
Since ive just recently picked up a sponsership i thought i might try and help you out.
1)You need some good, and more importantly consistent results behind you before you go out applying, a potential sponser wants to see somebody who will always do well, not just once or twice a year.
2) You need to be fast, you can't be losing races. You don't need to winning them all but if you are thats a bonus. In a higher catergory usually too.
3) This is the most important one, you need to be a nice person. Sponsers need you to be approachable and the sort of person who will have a chat to you in the shuttle line or the pits, where ever. A lot of sponsers would refrain from sponsering someone who is really fast if they're a total tosser whose up themselves etc.
A lot of hard work and persistance, its not going to happen overnight so you need to be doing plenty of training and getting to know the right people.
Good Luck