Before I move from one Exchange server to another, I want to let all the company's laptop users back up their mail. They're currently using Personal Folders (.pst) files. What's the best way for the users to back up their mail?

Perhaps the sole advantage of .pst files is that they're easy to back up and restore. However, they're fragile and they lack Exchange's ability to provide single-instance storage. If your users are using .pst files, they can put them on a file share on a central server, which you can then back up. When your new Exchange server is up, users can drag items from their .pst files to their Exchange mailboxes, putting the data back on the server where it can be centrally managed. Overall, the better solution is to use Outlook's support for offline file folders (.ost) files. The .ost file maintains a client-side copy of selected server folders. This technique gives you the best of both worlds: access to mail items while you're disconnected from the server and the management advantages of keeping the data on the server. You can't directly transfer messages from a .pst file to an .ost file, so your users must drag messages to their server-based folders, then synchronize them to the .ost file.