We wrote a an entire 7-part series on how to sell ads online. The main points were

   1. Have an outstanding product (i.e. audience) to sell.
   2. Hire a dedicated sales person.
   3. Provide a self service ad management system.
   4. Show detailed traffic statistics.
   5. Sell exclusivity.
   6. Let the market determine the price via an auction.
   7. Simplify the advertising process.

Give it a read - http://www.trafficspaces.com/blog/2009/12/16/how-to-sell-ads...