Setting up a bunch of specific signatures in Outlook makes for more effective communication, but there's no obvious method for migrating them when you move machines, since signatures aren't stored as part of the main PST file. Microsoft's Outlook Team Blog explains the secret of how to move signatures from one place to another: in short, they're stored as files in an obscure folder which you can access by control-clicking the Signatures button under Mail Format. While hardly obvious, at least this method makes signature backup possible.
How do I back up my signatures? [Microsoft Outlook Team Blog]