A few suggestions (I get about 500 emails a day, though a large portion of that is spam and much of the rest is mailing lists not personal emails)
1. If you do not already, use automatic rules to at a minimum pull out mailing lists and other subscriptions into separate folders (I use one folder for each mailing list and some catchall folders like "vendors" to stick general requested but not personal commercial emails, another for event notices/communications etc)
This should help keep your actual "inbox" to just personal emails and non-yet-auto-sorted mass emails.
(and at least much of the time as you get a new form of mass email, assuming it is useful, you should take a few seconds and set up the rule right then)
2. One of the most efficient email people I know uses his own software (Activewords - http://www.activewords.com) to give him a set of standard text he can auto-insert with a few simple keystrokes. He uses this for commonly needed replies (such as in his cas a request for a trial license or his fulfilling an offer for a free full license etc).
By using Activewords, or a bit less efficiently by setting up some templates and/or a library to cut and paste from of your commonly needed replies you can make at least faster the process of replying to many emails.
I use something similar as I manage the email communications re MeshForum (http://www.meshforum.org), the process of organizing a conference means I am sending the same info to many people, by using rapid text substitution I ensure consistency and save lots of time typing.
3. Another trick a sales person I know and highly respects uses is he blocks of time OFFLINE (very critical) during which he both composes emails to people and works his way through his inbox replying to people. By being offline while he is doing this, he avoids new incomming distractions, and finds he can efficiently and quickly catch up. I do something similar at least once a week (and regret it when I don't) during which I go through my inbox, file mail I have read and dealt with, and reply to mail which requires a response.
4. Having a series of checks & balances as well as fairly consistent ways of moving mail out of your inbox as you deal with it is very, very helpful. I try to file messages immediately after reading them (assuming I have dealt with them, or I flag them if I have to wait before replying). I have project folders as well as catchall folders for this purpose.
I also make use of Outlook XP's feature of saved searches to help me monitor for and catch important mail that gets autofiled outside of my inbox, or which gets backed up inside of my inbox (in my case one such search looks for any email with "meshforum" in it that is not sorted into my MeshForum folder already)
5. Some of the people I know who get even more email than me also use multiple email addresses to help manage their email, it all probably shows up into the same tool, but they have "public" and "private" addresses, the private addresses are never published on the web and only given out to family and friends - this can be a simple yet highly efficient way to auto-flag and deal with email from those who are closest to you.