"This is the final video installment in our three-part series on Optimisation Essentials for AdSense for content. We've covered the best-performing ad unit sizes and taken a look at how to design your ad units for good-looking ads. However, even if you have large, well-designed ad units, they won't perform for you unless your users can see them! So what can you do to ensure your ad units get noticed? Place your ads where users are likely to look."

http://adsense.blogspot.com/2008/11/optimisation-essentials-part-iii.html

http://www.youtube.com/watch?v=m3SGp7-hQo4