2009.01.09 concentrate:

Aside from my preternatural skills of procrastination, one of my biggest obstacles to writing is focus. Sometimes, you run across a piece of advice that speaks directly to you, and this is one of those times.

Don't research
Researching isn't writing and vice-versa. When you come to a factual matter that you could google in a matter of seconds, don't. Don't give in and look up the length of the Brooklyn Bridge, the population of Rhode Island, or the distance to the Sun. That way lies distraction — an endless click-trance that will turn your 20 minutes of composing into a half-day's idyll through the web. Instead, do what journalists do: type "TK" where your fact should go, as in "The Brooklyn bridge, all TK feet of it, sailed into the air like a kite." "TK" appears in very few English words (the one I get tripped up on is "Atkins") so a quick search through your document for "TK" will tell you whether you have any fact-checking to do afterwards. And your editor and copyeditor will recognize it if you miss it and bring it to your attention.

from a post by Cory Doctorow

I've not done much actual writing (mostly outlining and, more often, NOT outlining—see procrastination, above) but when I was attempting to write that sci-fi novel I started in 2005, I was horribly horribly derailed by researching the position of mars in the night sky, from the mountains of northern california. In my defense, it gave me dialogue i might not have otherwise had, but still… distracted! The above is great advice for countering this, which I hope I can actually put into practice.