Choose a well-defined and realistic topic (see "How to Choose a Topic for a Dissertation").
Apply for as many dissertation write-up fellowships as you can. Keep in mind that this requires a lot of early planning.
Set a reasonable deadline for yourself that takes into account whether or not you'll be working while writing. If it's an unrealistic deadline, you'll end up disappointing yourself by not meeting it.
Write down your deadline on a calendar.
Set shorter deadlines for each chapter, and mark these down, too.
Check the nearest deadline after each day's work to see how you're progressing and to remind yourself of what you still need to do.
Prioritize if you're up against a deadline; cut out that extra TV show or that leisurely chat on the phone. If it looks like you might have difficulty meeting a deadline, talk with your advisors about it as early as possible.
Remind your advisors to assist in keeping your nose to the grindstone. Some advisors, if not reminded in this way, will find ways to make your project grow instead of shrink.
Don't let the size of the project discourage you. Take it in small pieces, remembering the sage advice that a journey of a thousand miles begins with a single step.