WordPress

Manually

Go to wordpress themes folder look for single.php
search for

<?php the_content(); ?>

Add the section targeting codes before and after this line, making it something like this:
<!-- google_ad_section_start -->
<?php the_content(); ?>
<!-- google_ad_section_end -->

Many times your theme might not contain single.php then might be page.php or even if there is single.php then also it might not have the above code line in it.

Google Integration Toolkit (Compatible up to: 2.8)

Download this Plugin http://downloads.wordpress.org/plugin/google-integration-toolkit.1.1.2.zip

This version of Google Integration Toolkit plugin provides following features:

* Integration with Google Webmasters Tools (both verification methods are supported: meta tag and verification file);
* Integration with Google Analytics (with optional Google Analytics – Google AdSense integration);
* RSS/Atom Feeds tagging – you can track visitors coming from your feed using Google Analytics;
* AdSense Section Targeting – improve AdSense ads targeting

In Blogspot Blogs

Go to your blog >> then to edit template

Click for expand widget templates

Look for line

<div class='post-body'>

Add the section targeting codes before and after this line, making it something like this:

<div class='post-body'>
<p><!-- google_ad_section_start --><data:post.body/><!-- google_ad_section_end --></p>
<div style='clear: both;'/> <!-- clear for photos floats -->
</div>