Here’s a simple change to include the Wordpress sidebar on all single post pages

Edit the “single.php” template.

Change this line at the top:

<div id="content" class="widecolumn">

To this:

<div id="content" class="narrowcolumn">

and to the bottom of the file, add:

<?php get_sidebar(); ?>

above the similar call for the footer.

That’s it.

You can also make this change to the other pages that display without a sidebar by default.