[Simon Lloyd]

here's an example of the code seperated by the pipe (|)

<script type="text/javascript"><!--
google_ad_client = "pub-1234567890";
/* 728x90, created 9/26/08 */
google_ad_slot = "1234567";
google_ad_width = 728;
google_ad_height = 90;
//-->
</script>
<script type="text/javascript"
src="http://pagead2.googlesyndication.com/pagead/show_ads.js">
</script>|<script type="text/javascript"><!--
google_ad_client = "pub-1234567890";
/* 728x90, created 9/24/08 */
google_ad_slot = "1234567";
google_ad_width = 728;
google_ad_height = 90;
//-->
</script>

Your positioning is done like this: 1|5|12
so the ads will be shown after the first post then the fifth then the twelveth