Inside AdSense has an interview with Ivan, an SEO expert. His advice:
Use wide ad formats. Many publishers find the 728×90 Leaderboard outperforms the 468×60 banner by about 70%. I particularly recommend using the 728×90 Leaderboard, 160×600 Wide Skyscraper, and 300×250 Medium Rectangle.
Embed Google ads in your content, and place Google ads at the end of your articles, news stories and reviews.
Blend your ad colours into your website.