If you use Exchange Server, you've probably implemented a backup strategy to ensure that user mailboxes are backed up on a frequent, regular basis. But if your users store their data in personal folders (PST files), mailbox backup becomes the user's responsibility. The Personal Folders Backup add-in is an excellent tool your users can use to back up their data.
This add-in, available free from the Microsoft Download Center, creates backup copies of a user's PST automatically and at regular intervals. After the user downloads and installs the add-in, they simply open Outlook, go to File | Backup, and click Options to specify which PSTs to back up and the backup frequency.
Backing up PSTs should be just one of the steps you take to ensure the survivability of users' Outlook data. Also have your users back up their PSTs to a network share where you or a network administrator can back them up. If users back up their PSTs to their local hard disks, you should provide a means for them to either copy those backups to a network share or to removable media such as CD-R.