This post on Lifehacker details a Microsoft utility that allows you to back up your PST (Outlook Personal Folders Files). I currently save all of my PST files into a folder in my "My Documents" folder and back up the whole folder at least weekly. The nice part about this MS utility is that you can schedule backup operations within a specified time period to a specific location and even schedule reminders if you have not backed a PST file in a specified time period.
I had at one point hacked the Offline Files so that I could mirror my PST files onto the server, but it caused pretty significant synchronization problems especially if I lost connectivity throughout the day, so I don't reccommend doing it unless you have a bomb-proof, always-up network.
Whatever road you take, make sure that you back things up. Note that many of you may be using PSTs and not even know it, with the PSTs being stored somewhere in the Outlook folder on your system, so this utility might be just the answer for you.