Although this does not necessarily make the URLs shorter, it definitely makes them easier to look at...
Check and see if this will help: Ultimate SEO URLs

Example URL Transformations:

From: http://yoursite.com/index.php?main_p...products_id=24
To: http://yoursite.com/disciples-sacred...nked-p-24.html

From: http://yoursite.com/index.php?main_p...dex&cPath=2_20
To: http://yoursite.com/software-strategy-c-2_20.html

From: http://yoursite.com/index.php?main_page=contact_us
To: http://yoursite.com/contact_us.html
Hope that helps,
Brick