Your inbox says a lot about you. What does it say? That you waste precious time? That you lose sight of important messages? That you're unresponsive? Unfocused? Unprofessional? If you don't have a simple and consistent system for dealing with e-mail, you should be ashamed of yourself. Stop the madness, get your ass in gear and get a handle on it, man. Here's a simple system that (usually) works for me.
Aim to keep your inbox empty, or almost empty, at all times. This doesn't mean that you have to respond to every message, it just means that every message gets processed. The workflow for processing each message is simple. Practice it enough and it becomes second-nature, like muscle-memory.
DO - If the message is actionable and you can complete the action within two minutes or less, then do it immediately.
DELEGATE - Is someone else better equipped to handle it? If so, put it where it belongs - in their hands. Don't "pass the buck", but don't waste time with things where you can't add value.
DEFER - transfer the action to your TO DO list and then archive the message. This only works if you're managing a TO DO list effectively also. You are, aren't you?
DELETE - If you can't take action and there's nothing in the e-mail you might refer to later, delete it.
ARCHIVE - Anything you might refer to later and everything you've already processed. And don't create a complex folder system for organizing your archived mail. This wastes more time over the long run than what using your e-mail client's search tools will cost you. Categories can better than folders, they're more flexible because you can apply multiple categories to a single message. A limited set of folder or categories can help for a few things (e.g. personal receipts / professional receipts).
Three Times a Day - Max
Do you live in your inbox? Is that what you do for a living? Set three times per day, at most, to manage and respond to e-mail. Three times per day that you'll get back to "inbox zero". What helps me is to actually include a line in my signature that says "I check mail at 9:00 AM, 1:00 PM, and 5:00 PM CST". More than anything else, it's a reminder to me. When I see it, and the time isn't right, it reminds me to get the hell out of my inbox and get real work done.
Use E-mail Filtering Rules
Write e-mail filtering rules to pre-process messages. For example, messages for which you're only on CC might get a lesser priority that those for which you're in the TO line. Newsletters and unsolicited mail can go out of sight and out of mind until you have ample free time for processing them.
Unsubscribe, Unsubscribe, Unsubscribe
If you're not getting value from regular subscriptions, take the time to find the unsubscribe link and get off the list.
Teach People How to Treat You
Don't be a dick, but don't be afraid to remind people that it's not necessary to include you in direct replies, just because you were on the original CC list. If people are thoughtlessly wasting your time, you should not feel guilty about giving them a subtle reminder that you have better things to do.
The key to any system is consistency. When you slip out off the tracks (and you will), just put yourself back on. The more you do, the more the habit sets in. Keep coming back; it works if you work it. If you don't own your e-mail, your e-mail owns you. Don't be an e-mail loser. Take charge. Get a system (like this one), personalize it, and then stick to it.