Microsoft Outlook pst-file backup-up Add-in

The Personal Folders Backup download allows you to easily create backup copies of your .PST files at regular intervals.

Officially, this add-in only works with Outlook 2002/XP, Outlook 2003 and Outlook 2007 but can be made compatible with Outlook 2010 as well by setting a specific option in the Registry.

Update:
The Personal Folders Backup add-in is no longer available for download as all the related Outlook versions have been out of support for quite some time now and the add-in is not compatible with any currently supported version of Outlook.

As an alternative, consider making manual backups, use a similar add-in or script or one of the many other (semi-) automated ways.

Download: Outlook 2007/2003/2002 Add-in: Personal Folders Backup