[peetbee]

Try to look at it this way, what are you offering the potential sponsor?

In terms of nationals/regionals are you doing enough of them to offer value for money, ie 4 out of 6 regionals may mean that every round would count but if you get the results you could even win the region over someone who's gone to all 6! It's not just the results but the exposure that the brand gets.

Sponsors are interested in the promotion of their brand, so are you able to offer setup advice and other support to other drivers, are you friendly, approachable, well presented and prepared? (sponsors won't want to see you breaking the car due to crap race prep!)
This goes for forums as well as at club meetings in addition to other competitions.
As all of these may encourage sales over brands who don't appear as well supported.

Hope some of this may help - I've never been sponsored myself as I'm a rubbish driver!!

Edited to add: Dang, Ben beat me to it and in less words!!