I used to use folders, too. Then sometime last month I came across one of the pages on the web site for the book "Total Workday Control Using Microsoft Outlook." It listed 3 tips that are explained more fully in the book. One of the tips said this:
"Filing E-mails with Outlook Categories"
"One of the principles of the Total Workday Control system is that you should get e-mails out of you Inbox as soon as possible. Otherwise they clutter your thinking and your workday. But where do you put them? You should of course delete those that are useless, and convert others to tasks if needed. But the rest you should file in a way easy to find later. Most people who file mail create multiple personal folders with names based on topic, and drag e-mail into those classified folders. I do not like this approach because it precludes viewing old mail in one long chronological list, and if mail fits two topics you will not know where to file it. Rather, I recommend you create only one personal folder, drag all e-mail into that folder, and then assign Outlook Categories to that mail based on topic. You can then create a custom view that displays collapsible groups of mail collected by category topic (see figure at right). This has all the benefits of multiple folders but none of the disadvantages. It is a fantastic way to file mail by topic, and a great way to find mail later. This approach is described fully in Chapter 7 of the book."
http://www.workdaycontrol.com/3Secrets.html
I gave it a try, did mass categorizations of existing emails in various folders, and threw them all into one "Old Mail" folder. So far, it seems to be working well. There is a bit more work involved than with the previous "drag and drop into various folders" method, because you have to assign each email message to a category before moving it to the old mail folder. This takes a few mouse clicks (I haven't foind any keyboard shortcuts) that the "old" way didn't require. However, as the excerpt above describes, the benefits for me outweigh the small bit of extra effort required.