Unspammed
From the time I started using email (around 1998) until a few months ago, the amount of email I received daily increased from a trickle to an uncontrollable torrent. Today, however, I monitor a total of six email accounts from a single interface, and I'm able to do so with very little effort and none of the sense of overwhelm that used to be common. My approach borrows from popular philosophies like GTD and Inbox Zero, but it's not a complete implementation of any of them. I wanted to write it down in case anyone is looking for a pragmatic way to get the email flood under control.
The Front Line
I get almost no outright spam. My professional accounts arrive from my own mail server, which runs SpamAssassin, and my personal accounts are filtered by Gmail. Before I had these aggressive front-line measures in place, I spent an unfortunate amount of time manually filtering and deleting spam. Life is too short for that kind of crap, so if you still get spam, look into how you can insert a more effective filter to keep it away.
Removing Needless Distraction
There is a class of email that is vastly more insidious than outright spam. It includes newsletters, mailing list digests, marketing messages from retailers, and product updates from online services. These messages are dangerous because they are targeted. You asked for them at some point (or at least didn't opt-out). They're often addressed to you personally (via a mail merge program, of course). And they're almost certainly not valuable.
If you actually care about reading the latest activity on your Google Groups or Quora, or you care about those sales at Fab or Gilt, or you need that coupon for Express or Victoria's Secret, that's fine. But I doubt you need all of them. I realized I didn't need any of them, but I was getting several per day anyway. It's easy to pull the trigger and give someone your email address on the pretext of them sending you "valuable messages," but they do add up, and they do take time to process (especially if you enjoy "retail therapy").
You can undo all the damage by clicking the "Unsubscribe Now" link in the email footer. Doing this religiously over the course of a couple of weeks drastically decreased my volume of incoming mail.
Making Real-Time Notifications More Ephemeral
Unsubscribing doesn't mean you have to disconnect yourself entirely. I used to receive email whenever anyone mentioned me on Twitter or Facebook. Now, I use the push notification features of their respective iPhone apps to stay alert. If I don't want to be distracted, I put my phone face-down and don't touch it for a few hours. The notifications accumulate on the home screen, and if I don't see anything interesting, they're gone with a swipe.
In this way, I'm able to reduce the permanence of these urgent but often trivial interactions. They don't stay in my inbox distracting me and needing cleaning; they just come and go. If you need to keep tabs on a less-chatty social network like LinkedIn, put a recurring event on your calendar to process all of the notifications in one shot instead of being distracted daily.
The DMZ
The inbox is sacred. The inbox is my collection of things that need to get done soon.
If I receive a message that I cannot act on in the next 24 hours, it gets turned into an event on my calendar, and I handle it when that date rolls around. If it doesn't need to be acted on at all, but might be useful for reference, it gets tagged and archived. If it is not actionable and I can convince myself I can look up the information it contains when I need it (which is probably never), it gets deleted outright. Not archived, deleted.
The inbox is sacred. The inbox contains only the things I need immediate access to.
Getting to this point is one of the most challenging parts of instituting sane email practices. If you haven't kept your inbox pared down, it likely has years of miscellaneous stuff you don't want to part with, homogeneously blended with all kinds of stuff that is worthless. You stare at this several times per day, and it's easy to become numb to its distracting and stress-inducing influence.
The inbox is sacred. The inbox is not a junk drawer.
The answer is the DMZ (a term borrowed from the military, meaning "demilitarized zone"). Create a tag or folder named DMZ, and move everything currently in your inbox into it. Odds are good that you'll never need to look in there again, but it's still accessible to your search feature if you need to look up a license key, password, or receipt. From this point forward, you can try to use a more strict tagging scheme to improve how quickly you can look things up. I personally use very broad tags (Work, Read Later, etc.) and rely on search almost exclusively.
Getting down to business
If you do everything I describe above, you will have a perfectly timely and relevant inbox, and the things arriving in it will have a much higher probability of being things you actually care about. My email has once again become a tool to improve my productivity by collecting the information I need and presenting a concise list of what needs my attention. If your email is out of control, try one or two of these tips to make it your ally again.