Email Archiving in an Exchange Environment

GFI MailArchiver is the 3rd party product I see recommended a lot for this purpose.

And while this won't be of any help now, many of the features in Exchange Server 2010 will focus on easing archiving and retention.

Through the use of GPOs, you can manage what happens to .pst files. We auto-archive ours to the network where they're backed up regularly and it's easy enough to bypass them being saved to the local disk.

Archiving email is no panacea for problems endemic to users. That is their need to save everything. You have to practically beat it into their heads that there generally isn't a need to save everything. To that end we have Exchange delete everything in the Deleted Items folder (users were in the habit of using this as another folder and NEVER emptying it.)

Small reminders about cleaning up their email and how it results in efficiencies throughout the organization is helpful, etc., etc.

Edited to add:

Red-gate also has an exchange archiver and based on their other tools, this one should be top-notch.