There are some very slow people with nice sponsors, and some very fast people with them.
race results help, but its not the whole picture.
You need to ask yourself, what can i offer a sponsor that they dont already have?
The more you are after
(gear, discounts, cash, race entries, whatever
), the more you need to be prepared to offer.
Smaller shop or order discounts are available fairly easily, pro level discounts and free gear are much harder, and cash is be far teh hardest to get.
Keep track of your race results, your upcoming races, what you do in the community that may help the sport, etc, and put this together in a resume' like you would for any other job, a brief bio and what you do/who you are/why they should sponsor you,
your race results from the past 1-3 years, and the upcoming schedule for this year/next if you have it.
If you have them also list previous and current sponsors.
Late summer through new years is usually the best time to send letters off to potential sponsors.
Be prepared to not hear back from a lot of them, or for a very long time.