One thing that works well for me is making sure they are targetted as good as you can make them. As Zack mentioned keywords play a big role in this. Also excluding irrelevant text from the page will help get more focused keywords.

This is one of the problems I have with adsense. If you have a blog type site or more literary site, not to mention current events, you sometimes wish you had more ability to pick and choose. One reason I like amazon.