You need people to approach you with advertising. Beyond Barns and Noble, you won't be able to get any adds unless it is a high trafic site, with content similar to the advertisers topic.
I am working for a company now that gets thousands of hits a day, but can't find people to advertise at industry average. (about 100us a month.)
Though, if you charge say $10-20 a month, people will probably look at advertising.
Try this (though it rarely works)
Drop a site (a bussines) an email about your site, inquire if they want to take out a banner ad. Try also to make sure they are a US company if you are in the US ect ect. If they are not, conversions to other currrancys kill the company. I was trying to make a deal with an Australian company, but the dollar is 1/2 that of a US dollar. They wanted to advertise, but didn't want to pay $200/month aus.
Good Luck