It's all who you know blind emails/letters don't really produce results.
I'll bribe them $20
couldn't hurt to try though, managed to work at the beginning of this season too.
Keep riding and getting the word out about yourself. Make sure you get a reputation that you're easy to work with.
I don't want to get you down, but you're just not at a major sponsorship level of riding right now. Local shop sponsorship is about where you're at right now. But, that's pretty much where every sponsored rider starts out at, so don't let that discourage you.
I know this. Sending into companies to start getting known though is what i'm doing. I do all of the local competitions (rail jam and stuff) as well as doing nationally ranked competitions (i got 2nd in pipe out in Colorado at the beginning of this month). I know a couple kids who are a whole lot worse than me who got sponsored by doing this kind of stuff too so going by that I'll see what happens.