Tricks For Improving the CPC, Official Google

Make sure that the best performing ad unit (highest CTR) is the first in your HTML code to improve your revenue.

• First, you will need to place a DIV tag right below the BODY tag in order to let the browsers know which ad unit goes first in the source code HTML. Note that it doesn't mean that this ad unit will show up first on the page. Here is the code:
  
  <div id=”ad1″>
  Insert your ad code here
  </div>
• Next, insert the CSS code below into the style sheet of your website. Since all sites are different, we recommend that you test attributes (left, right, top, bottom) in order to place your ad unit correctly. Make sure to adjust height/width depending on the ad format.
  
  #ad1 {
  width: 468px;
  height: 60px;
  position: absolute;
  left: 140px;
  right: 0px;
  bottom: 0px;
  top: 350px;
  }
• If you don't use style sheets, you will need to insert the above code into DIV tags between the HEAD tags in the HTML source code. 
  
  <style type=”text/css”>
  Insert above css code here
  </style>