Sorting incoming email faster is one of the critical ingredients of
email mastery. You need to be able to process the contents without getting bogged down in doing everything.
First, know what you have. Second, do the right next thing.
Taking as few seconds per message as possible whip through your inbox and delete, file or label everything as appropriate.
If you don't already have labels set up for your mail, do that quickly now. You need very few:
1) "active project support" (or "task support" if you need a shorter term)
These are emails which you need in order to perform tasks for a current active project. If the task won't be done in the next 48 hours and the information in the email can be easily copied to your to-do list entry for that task, don't keep the email. If the task is coming up quickly, just apply the label and spare yourself the extra effort; strike a good balance between the pleasure of an empty inbox and busywork copying information around more than is necessary.
2) "waiting for"
Use this label as a reminder of something you may need to nudge someone else for or which is queued up behind another task you want to finish quickly.
3) "to read"
You will probably want more than one of these, for example, "to read: business" and "to read: personal". This label goes on anything you can tell from the subject line you don't need to read right now, but will need to look through at some point. Catching up on these categories can be a good task for when you are feeling braindead or have only a few minutes before you go into a meeting.
Bloggers may also find it handy to have a "to post" label for email to which they want to respond publicly or which inspires a post, but in general the 3 labels above should be sufficient to cover anything worth keeping in the inbox temporarily.
Use color for your labels to help you quickly recognize the categories of the past decisions the labels represent. I like to use a moderately dark color I'm fond of for "active project support" and a pleasing but limbo-implying color for "waiting for" (which for me are forest green and light purple, respectively). For tasks you need to draw yourself to more often than you might otherwise do them, use your favorite, most energizing color (in my case, bright spring green on my "to read: business" label to keep those newsletters from getting stale).
If your mail program allows you to list the label in a dark color with light text (e.g. Gmail) or otherwise make labeled messages really stand out from new or 'read but unlabeled' ones, do it. This approach makes it incredibly easy to tell at a glance - without needing to read the text itself and be distracted by it - what's in your inbox and that no processing action is currently required.
Why label instead of move to folders? It avoids the risk of "out of
sight, out of mind" while allowing you to tell at a glance that you've
already handled everything that currently needs handling. You get the
benefit of inbox zero without wasting a lot of time or having to
establish new rituals to check special folders.
For categories that you don't want to be reminded of until you're
performing a round of that activity (e.g. something like "to read:
professional development"), folders are helpful. In Gmail you can keep
those pending things labeled appropriately but archived, ready to be
retrieved by selecting all messages with that label (which you just
remove after reading). Gmail users are also encouraged to take
advantage of the latest controls for showing & hiding the list of
labels to keep only these primary ones in view in your sidebar.
Remember, when processing new email you want to be incredibly quick, spending as few seconds on each message as is necessary to answer the questions "does this require any action on my part?" and "if so, what?"
Most email can be immediately deleted (or thrown into a single archive). My approach (using Gmail) is to scan over the unread subject lines checking the boxes beside any spam and then clicking "Report Spam" to clear that out of my way. Then I scan down again - faster this time because I've already read the subjects once - checking the boxes for anything that can be archived without opening (e.g. "John Doe is now following you on Twitter") and click "Archive". What's left can be dealt with one by one: reading, forwarding with brief comments and usually an improved subject line to someone else if appropriate (delegate), noting a task or event for the future by copying information to my calendar or OmniFocus (defer), replying if it will take less than 2 minutes (do), and/or labeling if the email needs to be kept for the moment to support a future task (including any responses which would take more than 2 minutes).
The goal of the 2 minute limit on doing is to avoid duplicating effort on low-return tasks; you've just spent enough energy and time to decide the necessary action, so rather than having to remind yourself of that again later, do the fast actions now. Beware though; any "it'll just take 5 or 10 minute" emails really add up. Be firm with yourself about the 2 minute limit and come back to the longer tasks after you've finished processing. Remember: first, know what you have; second, do the right next thing - as opposed to the one that just happens to be next in the inbox.
Building this habit into your daily routine will change your relationship with email. Instead of a murky pit of unknown obligations, your inbox will be a functional space. Repeatedly throughout your day you will know exactly what's there, if anything, and what commitments it represents when it isn't empty.