Well after more thought on the subject, usually a contest has a prize, now money is really hard for some people to give up, even if it's their site, and such gift certificates, well, don't really count, unless you have to pay vendors, then it costs something, unless of course you also not only own the site but are also a vendor, uh oh.

So, enticements

1. get some popular vendors to front some prize-a-rella (one thing from my store)
   -the artist will obviously use it, which is advertising for that vendor
   -if the vendor is in agreement, that nulls any monetary tax issues on loss of cough virtual product and/or book-keeping

I mean, if every month, there is a monthly freebie, this isn't altruism in its barest sense, it's advertising cuz that page always reads: 'and for more great stuff, check out my store'

2. some kind of web badge/graphic to go on the artist homepage and/or MyFacedotcom, if this is about bragging rights, 'whadya want a medal? Fine, here' BAM!
   -maybe at the end of the year, these can translate into some form of gift certificate-ish thing, or if the winner is a vendor, a lighter house take on 1 item.

3. Bite the bullet and give house money(gift certificate) & honourable mention in the newsletter