First and foremost be professional. Have a good looking resume (or CV) and approach potential sponsors in a professional manner (be humble and grateful and confident in yourself).
Go to lots of potential sponsors, don't expect to get everything from one person/company.... seek out 10 and you may get 3 and then seek out 10 more. Some may give you more than others, but don't turn down smaller sponsors they may be the ones who are likely to stick with you for a long time. Be gracious with whatever they offer to do for you... I have had people approach me and I tell them what I am willing to do for them and they tell me that isn't good enough... well then i don't want to do anything for them ever... take what they offer and be grateful and talk them up just as you would someone who gives you much more. Something like a piece of clothing might not seem big, but as you tear your pants (or race suit) on a gate and they give you new stuff you will be real happy you have a good relationship with them!
think outside the box, go to your local sports stores and mountain where you train, but also go to your local bank, grocery store, local businesses and other such places.
Definitely look into sports organizations, boosters etc.
Be prepared to tell sponsors what you are going to do for them, where and how you will display their logo and other ways you can do things for them... do they have any events they need volunteers for? Again be creative.
Listen to other's suggestions and visit some high level racers' websites, see who sponsors them to get some more ideas on who to go to... and like Steve P says, get your parents to help on the bandwagon and throw that word "Olympian" around.... even future olympian gets people excited.
Once you get sponsors, keep them updated, shoot an email or a phone call their way to tell them how you are doing, how much you appreciate their continued support and what your plans are... when you give stuff to people it is nice to know they continually appreciate it!
that's all i can think of now... but this should definitely get you started!
Lastly, you need to get out there and promote yourself, don't think at this point that people are going to come running to you to sponsor you... get out in your community and tell them why they should want to sponsor you!
that's all i can think of now... but this should definitely get you started!
Good Luck!