1. Create folders and move EVERYTHING out of your inbox that doesn't require you to take action. That way, your inbox itself is the list of incompletes, and there's no additional clutter there. For example, if there's a newsletter or a blog post that you receive that you'd like to read later, either filter it into a "to read" folder from the get go, or move it into that folder. Something like a newsletter is more a "would be nice" than a "must respond" and doesn't need to have prime real estate in your inbox.
2. Unsubscribe to anything that you no longer find useful! If you can stem the inflow a bit, it will help decrease the feeling of overwhelm.
3. Delete excess daily (or at least archive it). If you've gotten into the habit of not deleting email, for years!, sort everything initially by "from/sender", and then delete or archive in batches.
4. Use NudgeMail, a free service which allows you to take any email and forward it to yourself to receive in the future at a specified date and/or time. Want to sign up for that class on March 23rd, since that's when enrollment opens? Forward yourself the email (via Nudgemail) and it'll come back to you when you need it. I love it for stuff that doesn't require immediate action, since I know it won't fall through the cracks! Once you trust it, you'll see your entire relationship with email change. Getting a reminder right when you need it (akin to an alarm telling you to go move your car for street cleaning!) can be really beneficial.
For more about Amanda and Urban Clarity, check out her website at www.urbanclarity.net.