I'm an extreme sorter and have had an interesting time reorganizing my boss's email patterns - she gets 500 emails per day. After spam. And requires that all of her email remain in Outlook (meaning transferring, say, emails form 2001 into an archive file is out). It's still an organic process, but the most effective, and most easily adopted by her, have been to:
1) Use folders to separate functional areas. For example: A Company or Work folder containing Contracts (with a subfolder for each active contract), Business Development (proposals/leads), and Personal Development (education and conference materials, receipts, etc). Outside of the Company folder is a Personal folder for non-work related emails.
My only rule of thumb is embrace the use of folders, but don't go crazy with the subfolders. It's one thing to separate your M&Ms from your Snickers and Dairy Milks, another to separate the colors of your M&Ms.
2) Categories suck. They are not labels or tags. They are deficient. That said, there's little else that can help you highlight/color emails except maybe flags in Outlook 2003. I have a rule set up to categories any email that is sent from other employees within the company, so they don't get overlooked.
Once that's done: Rules, Rules, Rules. I haven't found a limit. I've got all manner of highly refined Spam filters first, followed by News filters that move all the lists and newsletters and RFP announcements to a news folder and mark them as read (unread messages denote priority and require attention; news is optional - it's procrastination, not work). Then there is a rule for each contract filtering any email from the customer domain to the appropriate contract folder.
And of course I would say read Inbox Zero (specifically this one) and Email Zen and take what nuggets of goodness mean the most to you before proceeding.
