There's a really easy way to get things started'go look at one of your competitors. There are tons of ways to do this ? the easiest way would be to put your keyword(s) into Google and see what pops up.
The order of the results is based in good part on Google's PageRank, which has a lot to do with the number of other sites that link to a particular site.
Pick one of the results near the top and click on the link. Hover on a banner or text advertising box and see where the URL leads. Take note of the particular network such as Commission Junction, ClickBank, Linkshare, etc. and the particular programs on the site. I keep a list using Microsoft Notepad when I surf competitive sites for clients.
Most of the time I ignore CPM-based networks like DoubleClick or AdBrite, and contextual ad networks like Google and Yahoo. I do pay particular attention to the category affiliate network banners and text ads. For example, if you surf a few competitors and find a lot of banner advertising for web site hosting, that means your competitors are having at least some luck with website hosting affiliate programs.
When you come up with a good list of networks, programs and categories, go visit the networks that appear the most on the sites (try to go to 5 or 6 different sites so you can see any patterns that emerge).
You should now have a terrific list to start from, including a list of networks, programs and keywords. Now all you have to do is poke around and see which pay the best and try to figure out which your visitors might like.
While you're on the other sites, try to get a feel for where on the page the ads are placed, which can make a huge difference in clickthrough and conversion.
