One way is to be a great ambassador to the sport and to also have some decent results. Briefly, get a race and life resume together. Write a great cover letter introducing yourself. Add a few photos. And if you have the resources, you could even put together a short video (post this online). Via email, send that info in the fall to the companies you want to target.
I reach out to websites and get sponsorships. Often times, companies will pay me in products/goods (enough for me to eat my favorite food all day: http://secretmenus.com/chipotle/secret-menu/).
Map out how you will brand your name to gain sponsorship. Brands have loyal fans and you’ll want to begin building a public presence to attract your own supporters. Use social media, start a blog http://howtostartablog.com/ or build a website that documents your successes and training.
Good luck!