Blend your Ads. Select the background color of Adsense unit to be exactly the same color like your site background. Set the color of the title link of google ad unit to be similar to the link color on your site.

Avoid the 468×60 banner size. The google ad unit appeared on the bottom of this page doesn’t perform well.

Chose the 336×280 big size ad. My experience says that this big size ads has been the main source of my earning from Adsense.

Always avoid image ads. Image ads always looks pretty but somehow people seems to be reluctant to click on those type of ad. Therefore use strictly text ads for Adsense.

Put the ad unit above the fold. Trust me, 80% user of my site clicking ad unit which appeared above the fold. Unless you have really long and interesting post, never put ad unit below the page fold.

You could elaborate this basic tips further and remember to use multiple ad channel for adsense ad unit for better experimentation.

UPDATE

Recently Google adsense team gives us insider info about the strengths of each ad format:

* The 300×250 medium rectangle is a widely supported format that advertisers often use when designing their branding and rich media campaigns. Using this ad format and opting in to both text and image ads may generate more placement targeting opportunities, which will help drive up competition for your ad space and should lead to higher earnings over time.

* The 336×280 large rectangle tends to have a higher clickthrough rate due to its larger size. It often performs well on text-heavy pages where it’s integrated into the content.

Read more on Google Adsense Blog