The signatures for Outlook are stored in the folder:
C:\Documents and Settings\username\Application Data\Microsoft\Signatures

Now all you have do to is go to the folder and make a backup of all the files. Each signature will have three associated files (.htm, .txt and a .rtf file).

Simple if you know where to look.

Check out these posts too:

  1. Outlook 2010 – My Experience
  2. See what’s in your Outlook PST file
  3. Create Your Own Cross-Platform Backup Server
  4. Five Best Disk Defragmenters
  5. Outlook knows better?
  6. Delete Browsing History in IE7
  7. Change the location of the 'My' folder in Windows