Keith Robinson has written a piece 
at Lifehacker, offering “some tips about dealing with your e-mail and doing your very best to respond to every legit e-mail you get in a reasonable time frame.”
He covers the importance of making an effort, how to set up a file system (which looks wildly different from mine), and tricks for making sure that you get as close as dammit to the holy grail of an empty inbox every day.
He also features one of my favourite rules: “When in doubt? — DELETE. If you don’t need to keep something and don’t plan to respond, just trash it.”
Instead of trashing all mails I use a Mail Act-On rule to just move any doubtful mail to a folder called @All Mail. I borrowed the functionality from the Gmail interface.
This way I do have all the mails I might need to take a look at later for reference at hand while keeping my inbox extremely clean. After replying, moving in that folder and manual sorting I come to an empty mailbox almost every second day.
I thought this might be interested for one or the other.
About six months ago I started the tedious task of slowly reducing the number of mailboxes I’ve created and my dependency on them as a primary method of organizing and finding messages. About three months ago I dropped some of the baggage when switching from Mulberry to Apple Mail. Since then I’ve managed to keep my primary inbox from growing much but it’s long overdue for legacy cleanup.
It’s taken many years to develop a mental map of where my mail is, using mailbox names as reminders, so there’s a powerful urge to somehow preserve that level of message organization. Another part of me desperately wants freedom from those hundreds of mailboxes. There’d probably be some short-term suffering, but the potential for longer-term satisfaction with simplification is a strong motivating force to abandoning them. Just need to convince myself it’s really not that important and worth the effort keeping messages so diligently mailbox-organized anymore. I suppose it would be easier if I trusted Mail as much as Mulberry …