I found this great tip How To Display Sidebar in Single Post in WordPress 1.5 / 2.0 Default Kubrick Theme.

The only problem is that the code that you should add is not properly shown in the post. He just copied the xml code verbatim in the post without escaping it and the browser doesn’t display it properly. So I’m reproducing it here for your convenience.

1. Go to the theme editor. Appearance ⇒ Editor
2. Click on Theme Files ⇒ Templates ⇒ Single post (single.php)
3. Change the class of the first div to be "narrowcolumn" instead of "widecolumn"
That is, replace:

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

with

<pre lang="xml">
<div id="content" class="narrowcolumn">

4. Add a call to get_sidebar() at the end just before get_footer()
That is, add the following line:

<?php get_sidebar(); ?>

before this line:

<?php get_footer(); ?>

Hope this helps. Remember that it was Angsuman Chakraborty who came up with the solution. I just merely reproduced it here because I noticed that xml code in the original post is not readable.