When I’m working on a story or novel, I set a modest daily goal — usually a page or two — and then I meet it every day, doing nothing else while I’m working on it. It’s not plausible or desirable to try to get the world to go away for hours at a time, but it’s entirely possible to make it all shut up for 20 minutes. Writing a page every day gets me more than a novel per year — do the math — and there’s always 20 minutes to be found in a day, no matter what else is going on. Twenty minutes is a short enough interval that it can be claimed from a sleep or meal-break (though this shouldn’t become a habit). The secret is to do it every day, weekends included, to keep the momentum going, and to allow your thoughts to wander to your next day’s page between sessions. Try to find one or two vivid sensory details to work into the next page, or a bon mot, so that you’ve already got some material when you sit down at the keyboard.
Leave yourself a rough edge
When you hit your daily word-goal, stop. Stop even if you’re in the middle of a sentence. Especially if you’re in the middle of a sentence. That way, when you sit down at the keyboard the next day, your first five or ten words are already ordained, so that you get a little push before you begin your work. Knitters leave a bit of yarn sticking out of the day’s knitting so they know where to pick up the next day — they call it the “hint.” Potters leave a rough edge on the wet clay before they wrap it in plastic for the night — it’s hard to build on a smooth edge.
–Cory Doctorow, http://www.locusmag.com/Features/2009/01/cory-doctorow-writing-in-age-of.html