I used to have rich hierarchies of folders for storing work email messages, but now I rely primarily on just two - Keepers and Purgatory. The vast majority of messages I receive at work are either (a) messages I can scan and delete immediately, (b) messages I wish to keep for reference forever, or (c) messages I wish to keep for reference but which have a limited "shelf life". The realization that this third "Purgatory" category exists has helped me prune down my inbox tremendously. I found that I wanted to keep many messages with information that was useful over a span of weeks or months, but that after that period, the information was "stale" and no longer needed. I now drop such messages into the Purgatory folder, which has a simple Outlook archive rule - delete all messages older than 6 months. This sliding 6-month window lets me keep messages while they are useful and prunes out those that are not.
A final note - a number of new tools are arriving to help people organize and find their email, one of which is Xobni, an extension for Outlook which uses social connections gleaned from your email to help you find messages and information about contacts. The Xobni Insight extension beta recently went public, but I had a chance to participate in the private beta a few months ago. My verdict? Although the organization by social context was cool and the email stats were spiffy, I still found my self gravitating towards Google Desktop to find messages. If I wanted to find a recent message from Joe, I found it faster to hit CTRL-CTRL to bring up the Google Desktop search box and type "from joe" rather than find the contact in the Xobni sidebar in Outlook.