//This will place your ad block inside your first blog post. AdSense’s heatmap shows the area within the content to be an optimal area for the display of ads. I found this solution in the WordPress forum.
//Place:
<?php $showAdSense = True; ?>
//before the line:
<?php if (have_posts()) : ?> <?php while (have_posts()) : the_post(); ?>
//Then insert this into the part where you want the ad to appear.
<?If ($showAdSense) { ?><div style="float:right;padding:3px;"> Ad sense code here </div><?php $showAdSense = False;} ?>