From the amazing Matt Quick:
https://mattthequick.wordpress.com/2015/01/15/how-to-automatically-back-up-your-pst-in-outlook/
Automatic PST backups for Outlook versions 2002, 2003, 2007:
http://www.microsoft.com/en-us/download/details.aspx?id=9003
Can be made to work with Outlook 2010:
http://support.microsoft.com/kb/2030523/en-us
And something new for Outlook 2013: