Working Smart: Taming Your E-Mail Inbox, Tip #2: “The most unproductive thing you can do when it comes to e-mail is to read the same messages over and over again. This has the effect of doubling, tripling, or even quadrupling your workload. Instead, you should read each message once, then decide what to do with it. Read-decide. Read-decide. This is the pattern of effective e-mail processing.”
p>A nice, straightforward article on how to use Outlook more effectively for applying GTD to emails, though most of the ideas will work just as well for any mail program.