Mine is just hand-coded, I'm not familiar with any sort of standard script. Wordpress does have a plugin of some sort that does something like this, but I like to have full control. Here is what I use, and I put it at the beginning of a post in the main index template so that it is injected right in the content.
<?php if (substr(getenv('HTTP_REFERER'), 0, 22) == "http://www.google.com/") {
echo "<div style=\"float:left; margin-top:1em; margin-left:1em; margin-bottom: 0.5em;\">";
echo "
<script type=\"text/javascript\"><!--
google_ad_client = \"pub-xxxxxxxxxxxxxx\";
google_ad_width = 300;
google_ad_height = 250;
google_ad_format = \"300x250_as\";
google_ad_type = \"text\";
//2007-07-01: Google Referrer Inside Content
google_ad_channel = \"5003101554\";
google_color_border = \"FFFFFF\";
google_color_bg = \"FFFFFF\";
google_color_link = \"A83000\";
google_color_text = \"000000\";
google_color_url = \"E6E6E6\";
//--></script>
<script type=\"text/javascript\"
src=\" http://pagead2.googlesyndication.com/pagead/show_ads.js\">
</script>";
echo "</div>";
}
?>
Make sure you keep all the slashes and quotes in there or it won't work.
To see how it works, if you click on my link at the top of these search results: how to become a millionaire - Google Search (http://www.google.com/search?q=how+to+become+a+millionaire)
you'll be taken to my page with the ad inserted. That inserted ad is by far my highest earning ad unit. eCPM is about 10 times that of the other ads.