Google Adsense, the tool for Google Publishers, has a really neat tool which gives people advice on the best locations to place Google Adwords on their own sites. The Google "Heat Map" displays areas on a sample page layout, with strong orange colours depicting the best performance areas and weaker orange colours poorer areas. Here is what Google has to say:
"All other things being equal, ad placements above the fold tend to perform better than those below the fold. Ads placed near rich content and navigational aids usually do well because users are focused on those areas of a page."
Other Optimization Tips for Google Adsense are that wider ad formats outperform taller ones as they allow people to read ads more comfortably.
Google reckons that colours that stand out without overshadowing your content are better than those that blend in with the site. Google says that whatever you do, the colour scheme should complement your site and here's an interesting tip:
"Also, rotating color palettes is a simple way to add variety and freshness to your ads. All you need to do is hold down the Control key and select up to four color palettes when generating your ad code in the Ad layout code page of your account."