Another problem is Google’s “smart pricing.” Being smart priced means that your click-through rate (CTR) is low and the money you earn per click is divided by between 2 and 10. For example, if a click would normally earn you $1.00, with smart pricing it could earn you as little as $0.10. Painful, isn’t it? Happily, this solution displays your AdSense ads to search engine visitors only, which means more clicks and a higher CTR.

The solution.

1. Open the functions.php file in your theme.
2. Paste the following code in it:

   function scratch99_fromasearchengine(){
     $ref = $_SERVER['HTTP_REFERER'];
     $SE = array('/search?', 'images.google.', 'web.info.com', 'search.', 'del.icio.us/search', 'soso.com', '/search/', '.yahoo.');
     foreach ($SE as $source) {
       if (strpos($ref,$source)!==false) return true;
     }
     return false;
   }

3. Once done, paste the following code anywhere in your template where you want your AdSense ads to appear. They’ll be displayed only to visitors coming from search engine results:

   if (function_exists('scratch99_fromasearchengine')) {
     if (scratch99_fromasearchengine()) {
       INSERT YOUR CODE HERE
     }
   }

Code explanation. This hack starts with the creation of a function called scratch99_fromasearchengine(). This function contains a $SE array variable in which you can specify search engines. You can easily add new search engines by adding new elements to the array.

The scratch99_fromasearchengine() then returns true if the visitor comes from one of the search engines containing the $SE array variable.

Sources: 1, 2, 3