There have been a number of posts here asking for tips on how to manage, archive, and search e-mail messages as part of a GTD workflow. I've found a pair of Outlook add-ins that make this process a lot easier. A review of EZDetach and MessageSave has been posted to my blog if you'd like more information.
In a nutshell, EZDetach lets you remove attachments from your e-mail messages but creates a link between the message and the attched file(s). MessageSave allows you save an e-mail message, or an entire folder of messages, on your local hard drive or network share in either native Outlook format or as text files.
I've been able to significantly reduce the size of my Outlook .pst files by detaching files and archiving messages. By using the Lookout search add-in for Outlook and setting it to index my saved messages, I still have full search and retrieve capabilities.