Efficiently backup PST Outlook files?

In addition to the 'usual' Auto Archive that takes mail off the Exchange server or main inbox but only to a single archive file, I do manual archives (File > Archive) per date, e.g. one PST per trimester. These will then not change at all, and I can even avoid opening older PSTs if they're not needed. You can probably write a macro for this, as there is no built-in feature to do it in Outlook 2007 or 2010.

See this Microsoft support article (same applies for Outlook 2010):

Outlook does not have a built-in function to split a .pst file into multiple .pst files. Therefore, you have to use one of the following steps to split a large .pst file:

1. Use the Archive feature to move items to a new .pst file
   • To do this, follow these steps: In Outlook, select your .pst file in the navigation pane. On the File menu, if using 2007 click Archive; if using 2010 click Cleanup Tools, then Archive.... In the Archive dialog box, click to select the Archive this folder and all subfolders check box, and then click a folder from the list. In the Archive items older than drop-down box, click a date. Click Browse, specify a path and a file name for the new .pst file, and then click OK.

     Note The new .pst file is displayed in the Mail Folders list as Archive Folders. If you want to change the display name for this .pst file, follow these steps: Right-click Archive Folders, and then click Properties. In the Archive Folders Properties dialog box, click Advanced. In the Personal Folders dialog box, enter the new display name in the Name box. Click OK two times to return to Outlook.

2. Use the Move to Folder feature to move items to a new .pst file.
   • To do this, follow these steps: On the File menu, point to New, and then click Outlook Data File. In the New Outlook Data File dialog box, click Office Outlook Personal Folders File (.pst), and then click OK. Specify a path and a file name for the new .pst file. In the Create Microsoft Personal Folders dialog box, specify the display name of the .pst file in the Name box. This is the name that is used by Outlook in the Mail Folders list. In the folder that contains the items that you want to move to a new .pst file, click one or more items to move. On the Edit menu, click Move to Folder. In the Move Items dialog box, click New. In the Create New Folder dialog box, enter a name for the new folder in the Name box. Under Select where to place the folder, click the new .pst file, and then click OK. In the Move Items dialog box, make sure that the new folder in the new .pst file is selected, and then click OK.