About 6 months ago, I picked up a copy of David Allen's "Getting Things Done" based on good word of mouth. At the time, I was disorganized to the point where I was bumping up against the Peter Principle, having been promoted to my level of incompetence. I had well over 1,200 e-mails in my inbox.
The main philosophy of the book is that disorganization creates a constant hum of mental stress. If you aren't absolutely positive that you have a system that will remind you of the things you need to do in time to do them, then your brain will struggle to hold on to them. Ever lay in bed at night thinking about all the stuff you need to do tomorrow, and thinking about how you'll remind yourself of those things? I did. Ever get out of bed because it pops into your head that you owed a deliverable today and forgot to send it? I did.
The book proposes an entire complicated system, but you can adopt it in simple form, as I did. I have since cleaned out my inbox. I have 2 "to do" folders (personal and work), 1 "for review" folder, and 1 "waiting on someone else" folder in my Outlook. I also have a ton of archival folders for things that aren't actionable. Every to-do I have gets converted into an e-mail, and I'm now confident that I have a full record of my to-dos that is accessible at any time. My inbox stays at zero. All e-mails either get put into one of my four action folders, deleted, or archived. If it's a thirty second to-do, then I just do it right away and then archive or delete. That's much easier to do when you don't have 1200 e-mails in your in-box. It's made a huge difference on my productivity.
There's a lot of other great stuff in there, some of which I'm doing, but that's the single thing that's made the greatest difference. I can't recommend the book highly enough, and I think it's going to be directly responsible for my next promotion.