An Inside AdSense tip tells people to try rotating in and out the AdSense blocks with non-ads. Google recommends Related Links. On being launched, the related links were confused as other AdSense product. So, rotating the ads out with real free content could be a better idea.
Read WebmasterWorld thread discussion on this. "And if you're feeling especially creative, try alternating between Google ads and Google Related Links in a given space on your page. This will encourage people to look in that space for interesting dynamic content."