Microsoft has a utility, called pfbackup.exe which allows you to automatically create back-ups from your .pst files.
However, when you install it it does not seem to work in Outlook 2007.
Here's the instruction on how to make it work:
1) Download the pfbackup.exe file from Microsoft's website (http://www.microsoft.com/downloads/details.aspx?FamilyId=8B081F3A-B7D0-4B16-B8AF-5A6322F4FD01&displaylang=en)
2) Use WinZip or a similar file extraction tool to extract the files in pfbackup.exe, launcher.exe and pfbackup.msi.
3) Double-click pfbackup.msi and follow the prompts to install it.
4) In Outlook 2007, choose Tools | Trust Center and choose Add-ins from the navigation list on the left. At the bottom of the Add-ins screen, under Manage, select COM Add-ins and click Go.
5) On the COM Add-ins dialog, click Add and navigate to the C:Program FilesMicrosoft OfficeOFFICE12ADDINS folder (if you are using a different language, ..ADDINS will have a local name (e.g. in Dutch: invoegtoepassingen)). Select outbak.dll and click OK. When you return to the COM Add-ins dialog, you should see a new entry, Outlook Backup Addin, with its check box checked.
6) Click OK to close the COM Add-ins dialog.
7) In Outlook 2007, choose File | Backup (Dutch: Bestand | Reservekopie) and set your backup options.