Part one: Neglect everything else.
Part two: Get disciplined. Learn to rush to your laptop and open it up. Open the file without asking yourself if you're in the mood, without thinking about anything else. Just open the file: and then you're safe. Once the words are on the screen, that becomes your distraction.
Of course, it's not distraction—it's work, and it's wonderful when it goes well. I'm sure other, more disciplined people can do it without needing to rush, but I have to. The moment you think okay, it's work time, and face down the words, you rush past all the other things asking for your attention.
Part three: Keep the Apple homepage, because it's rather boring. If your homepage is the website of your favorite newspaper, you've had it.
Just remember, this is how you earn a living. Really hardworking people at the publisher's are relying on your next book for their bonuses, to feed their kids, pay their mortgage. You owe it to them not to let years fritter away fruitlessly. First and foremost, of course, you owe it to yourself, and you owe it to your book—but if that isn't getting the job done, remember that it's other people's livelihoods on the line as well, not just yours.
These are just some of the sticks I use to beat myself into opening up the file. Once I do, I'm safe. I'm home free.