Please note: I’m assuming you still use the default theme. Things will be slightly different with other themes, but in general the same principal should apply.

In the file header.php remove media=”screen” from the following line:

<link rel="stylesheet" href="<?php bloginfo('stylesheet_url'); ?>" type="text/css" media="screen" />

In style.css, just before the line

/* Begin Typography & Colors */

Add

@media screen {

At the bottom of the file add the following line:

}
@media print {
    /* style sheet for print goes here */  
   #sidebar { display: none; }
}

Bingo no wasted paper from printing the side bar.

Well I’ve done something about it (well at least on my blog) – I’ve changed my theme so that on browsers that supports css and media types the sidebar shouldn’t print out. Here’s how you can add this to you’re WordPress blog

1. Create a file print-style.css in the directory for you’re current theme with the following content
   .dontprint{ display: none; }
div#menu{ display: none; }
2. In wordpress, goto Presentation, Theme Editor and select the file Header
3. Just bellow the line setting the css style sheet – should be something like this :
   <style type="text/css" media="screen">
@import url( %lt;?php bloginfo('stylesheet_url'); ?> );
<style>

   add the lines
   <style type="text/css" media="print">
@import url(<?php bloginfo('template_directory'); ?>/style-print.css);
</style>
4. Hopefully this should be enough, but if the theme author hasn’t used the div’s in the sidebar file you may have to add this line <div id="menu"> to the top of the file and this line </div> to the bottom of the file