Sending out hundreds of letters is one way of doing it.
Probably a better way is to target a few, but specific companies who are more likely to get involved.
Do your research on each company and personalise your proposal to each one.
Just because your racing may cost £5,000 doesn't mean you can offer £5,000 worth of advertising. Companies are looking for value for money, not to cover your expenses. e.g. You wouldn't sell a £10,000 bike with £5,000 of extras for £15,000, probably more like £12,500 coz thats better value for money.
A good way of approaching this is to get hold of your local media. Newpaper editor, radio station and ask them to cover your season or at least do an editorial with photo's etc. in return for a sticker on your bike. Space in their paper is NOT like asking them for cash so they will more than likely agree. (It also saves them hunting for stories about Hilda and her Geraniums)
Once they have agreed, you can then explain to potential sponsors that you can guarentee them exposure in the local paper etc. Even local papers charge a fortune for advertising, so a nice article would provide great value.
You could also get a business to business deal going, whereby each sponsor offers something to your other sponsors (on top of giving you cash!!).
So a gym may offer 10% off membership to your other sposnsors, the local tyre shop offers 10% off to the gym, the photocopying shop get 10% off the gym and tyres, and offer 10% off printing etc.
Get a good link going and you're able to offer a hell of a lot more than just a sticker on your bike and a link on a website.
Good luck
[Edited on 4-3-2005 by domskidue]