For some time now I've implemented a very basic form of email management. The core component of this was a simple configuration change I made several years ago - I reduced the frequency at which my email client checks for new messages on the server. I did this because I found that I'd be in the middle of dealing with one email, when others would turn up, creating an inevitable distraction.
I found that a polling interval of around 15 minutes worked well, especially in a very busy environment. These days at my (more relaxed) home office I have it set to 5 minutes, but I've found a problem recently.
It's been caused by my phone. Having finally upgraded to a smart phone, I now get email notifications almost immediately, which has the effect of undoing my basic management. So every now and then, I'll consider switching off the phone email notifications, and rely on the PC. Like in the old days.
Here are some other ideas that can, either individually or in combination, help you to manage that inbox:
Filter by Receiving User Name
If you have your own domain, set up individual email addresses for each mailing list that you subscribe to. This allows you to do two things. Firstly, you can easily write incoming mail rules to process these messages - depending on how busy you are, you might want to route them immediately to another folder to look at later. Secondly, if one of the services you subscribe to has its mailing list hacked (as if that would ever happen) you'll know immediately which one it is - and can act accordingly by updating your filter rules to accept the message only if it is FROM who it is supposed to be from (get this information from a legitimate message).
This approach also comes in handy for dealing with messages from Twitter, or LinkedIn. If you've set your Twitter email address to be, for example, firstname.lastname@example.org, you can write filter rules to process these directly into a folder to skim through later when you have more time. You might want to make an exception for direct messages though - just check the message subject for "has sent you a direct message on Twitter".
Filter by Sending User Name
You can filter either by the person sending, or the company they work for, based on your current priorities - for example, if you have a client project nearing a critical milestone, you could flag all incoming emails from that company as soon as they come in, so that they stand out on your screen.
Filter by Topic
This works in a similar way, but you do have to write rules based mainly on the subject line - which can be notoriously variable due to differing uses of wording, abbreviations, spelling, and mistypes.
Maintain Multiple "Outstanding" Folders for Different Projects
This is something I've been doing for quite some time, especially as I am often involved in voluntary projects for not-for-profit organisations - when emails come in relating to these whilst I'm in the middle of fee-paying work, it makes sense to defer them and store them all up for reading and acting on in one single session.
A dedicated folder is ideal for this, and naming them "0utstanding" rather than "Outstanding" i.e. with a zero rather than the letter O, keeps them at the top of my folder list.
Limit Your Use of the "Reply All" Button
As Eileen Brown points out in a recent blog post, Reply All emails make you less productive, so it's worth asking yourself each time you go to click that button if it's really something you want to do - how much will you be adding to your inbox by doing so?