Haven't taken a look yet, but I would suggest using Passionfruit. It makes it so all you have to worry about is promoting your spots rather than managing them manually.
Go to getpassionfruit.com
I'd also put your blog stats up in the Blog and Sponsor group, advertise there about your traffic, ad opportunities, and any perks. Then start by doing a few ad swaps (can still be free with Passionfruit coupon codes). Once shops and other blogs see the traffic, they may return as paying customers.