Adsense Optimization Tips
Optimum Number Of Ad Units
According to Adsense policy, you may add up to three Adsense ad units on each page of your website. This may not always be a very good idea though. For niches which do not have too many advertisers, adding three ad units may result in some ad units being empty or displaying PSA ads. Also since multiple ads are displayed this means that low paying ads are also displayed on your site and they have an equal chance of being clicked. This could mean in average earning per click going down. Some webmasters have reported that having only a 268 x 60 half banner have given them better results than having multiple ad units. But as with all other optimization techniques, this clearly depends on your website layout and niche.
In case you have implemented multiple ad units, note that they are filled in order of appearance with ads. So ensure that the most visible ad unit location is loaded first because this will have the highest paying ads.
Section Targeting
This feature lets you control, what ads appear on your site. Using tags similar to HTML tags, you can instruct the Adsense crawler to use only selected content to decide which ads appear on your site. Even though you can influence which ads appear on your site, note that it is against Google Adsense TOS to mislead the crawler to display ads not relevant to your site.
Use this feature if you feel that non-relevant ads are being displayed due to various non-regular pieces of content on your web page.
Also note that section targeting is not instant - it can take a few days for the changes to take effect.