I'm sure you've seen the posts on Slashdot, BoingBoing, and myriad other places that distraction lowers your score on an IQ test more than lack of sleep or chemical impairment.
Of course, studies like this always end with a list of things that we should do -- turn off the music, shut the door, get a quiet working environment, and so on -- but there's one that's missing that I think's really important.
Have a workflow that doesn't have any big pauses in it.
My computers at work (both of them!) are old enough, and slow enough, that I have to wait for windows to open and tasks to complete and so on. Every time I click a link in my stupid CRM webapp that opens a new window, I have to sit there and wait for it. I wait for Firefox to open the window, then I wait for the app to populate it with 12 or 15 HTTP queries, each of which go through my office's ridiculously small pipe. I seriously think that the router that connects us to the rest of the WAN is hamster-powered. Anyway. My point is that this takes so much time that I have time to get distracted. And then it takes me a long time to get back on track.
So eliminate those pauses! clear the clutter out of your workflow the way you clear off your desk. Because it's slowing you down way more.