Don't sell cheap adspots. Approve people that aren't idiots (Ask for a refernce; i.e.; a written sample of their work, or a site they've contibuted to). Let them post at will (within your rules and your "niche"), and just have your ads running as they type away and rant about shit.
You might make "more" money selling a couple of $25 blog posts every now and then, but if you can get a couple hundred people freely and consistantly creating content for your site, you might end up having a product worth advertising on.
Just
Nothing more.