I'm using the word "inbox" here, but I am not just referring to email. I'm referring to everything that represents "what you feel the need to get done."
The biggest leap in "productivity" I've made is, at least once a day, making sure my email inboxes (work and personal) are at zero. That means everything is either filed for later, actioned, or deleted. While that takes care of a lot of the "inbox," it's not all. There's the Google Reader. There's my physical whiteboard. There's Twitter. There's my podcasts, both in iTunes and on my Nokia phones. There's probably some other things I'm not thinking of.
There are some things I have to do more short-term, for example study for my CISSP test in a couple of weeks. I was never that great at studying for tests when I was in school. It's been years since I've had to take any sort of test that required studying, so what skills I had are rusty at best.
Then of course there's all the places I filed things "to do." When will I get to those? Do I care? It feels like the list never ends, but I take solace in the fact that I get what I can done each day. That which doesn't get done will still be there tomorrow. If I misprioritized something, or accidentally deleted an email I shouldn't have, if it is truly important, it will likely be sent to me again.
I don't follow any of the official "organizing" systems like David Allen's Getting Things Done or the stuff I learned in the 7 Habits class. But I do try and minimize the number of inboxes, make sure to leave everything in as clean a state as possible so that I can clean it again the next day. Periodically, I figure out which things I am reading/doing are no longer useful and remove them from the stream.
Speaking of that never-ending inbox, time to get back to it. Or to bed. Whichever.