Google AdSense & Contextual Advertising
The last two strategies for maximizing Google AdSense income discuss Interest-based ads and Google's Ad Planner Publisher Center. Don't worry if you never heard about those features. It's easy to participate.
Strategy #13) Consider Interest-based Ads.
If you choose to offer Interest-based Ads (currently, the default is to show these ads), then Google places a cookie on your visitor's browser, and then looks for that cookie every time they visit a site with AdSense ads on it.
If that cookie shows that your visitor is very interested in American football (based on preferences he sets in the Ads Preferences Manager, plus the number of football-related Web sites with AdSense ads, plus the number of football YouTube videos he watches), for example, even though he's visiting your site about green energy, he will see some ads about football. This increases the likelihood that your visitor will click on an ad.
Note, however, that if ads based on your keywords have much better cost-per-click rates, you may want to deactivate this feature. Fewer clicks, each worth more, may be more valuable to you than a lot of low paying clicks.
You'll find the Interest-based Ads preference in the My Account tab after you log into your AdSense account. You must also change your privacy policy to reflect that Google is placing a cookie on your visitors' computers. Even if you opt out of Interest-based Ads, you still need to update your privacy policy.