Some very good tips for those of us obsessed with a clean inbox.
Merlin has a series of posts at 43 Folders entitled "Inbox Zero".
I also second Scott's new rule: move everything you are copied on to a "Maybe" sub-folder. I've been using a similar rule myself at work for awhile now, where e-mails I'm copied or on from information-only lists get treated differently from those addressed to me. If you're not on the To line, e-mail etiquette suggests there is nothing for you to do - so why include this e-mail in those which have to be processed?
That ties in, I guess, to e-mail ettiquette - when replying to a list of people, put everyone you expect to take action on the To line and everyone else on the CC line. If everyone did this the office would be a better place...
Remember: Is it an action item? Do, Delegate, or Defer. Otherwise, File or Delete.
Related: Implementing GTD in Outlook 2003.