Ad Colors - Pick colors that blend well with the site, matches the background color and compliments the site. Make them feel like a part of the site. There are examples where customers changed from a blended background to a yellow or blue background and clicks dropped by more than 60%. Also, beware of Ad Blindness. If an ad sticks out too much the reader will immediately identify it as an ad and then pay it no more attention.
 

Ad Formats - According to Google the top three formats are the 336x280 that you see on the right above; the 300x250 medium rectangle; and then the 160x600 wide skyscraper. Wider ad formats typically do better than their thinner counterparts, partly because they are more noticeable and partly because they actually take up fewer lines. With every additional line of the ad you have a chance of losing that interested user.

Ad Location - The middle of the page above the fold location is typically best. Also according to Google if you have an article page with a long body of text, the bottom of that article can actually pretty successful.

Image Ads - Google suggests that if you want to maximize revenue you should consider turning them on. However, do this carefully as they can sometimes be too distracting. Thus while you may increase the amount you make per click and you percentage click through ratio you may ultimately see a decrease in page impressions over time.

AdSense For Search - You can use it as a search box for your site. You then receive payment for any ad clicks.

Link Units - These can be very useful since they don't take up much space, and they also allows the user to refine what they're interested in.

Experimentation - This is a theme echoed by all experts. Use Google's channels to test different positioning, ad formats and colors to find out what works best. Just by finding the right color, position and format combo you can more than double your revenue.

Keep your site unique - If you can keep the content of your site unique then you are likely to attract more visitors and thus more viewers for your ads. [try to follow On Page SEO to get better result and better serve for your ads]

Use high paying keywords - Some keywords pay much higher CPC than others. For a good list of reported (unofficial) highest paying keywords go here.

You can search for more adsense tips here.