As a former maketing manager in the industry I have to second the what's in it for the sponsor.
Think outside the square as well.
Can you take a women's MTB skills class for free for the store?
Make sure you provide race reports and photos (which are taken by yourself or a friend so won't have copyright issues).
Write an article or two for their website/newsletter or something?
Advertise their brand anwhere you can- putting a sign in your parked car at races, wearing a polo with their logo off the bike, referring as many people as possible to them?

Basically what advertising can YOU get for the store that you can access as being a sponsored rider- local paper article, weblog or anything like that.

I turned people down as soon as I saw "I need a bike and clothing and will ride for your store", umm hello? Any old chump can do that for me, so what makes you special/different and value for my money? Cas often I don't have a lot marketing money and so many people come with their cap in hand.