One thing that I like about Cameron's The Right to Write is the way it talks about grabbing time. You can't wait for a huge block of time to arrive in which to write, she says; you have to grab time away from whatever you're doing to write. She talks about the time lie: "I'd write a novel if I had a year."
I think her advice is even more relevant for those who are not making a career out of writing. A dedicated writer is going to find time, because the writer wants to write and is desirous of writing. A dissertation writer, however, might have many things he or she would rather do, and might have to do. For example, a research-oriented dissertation writer might be much more interested in doing more research (this, incidentally, is a bad reason not to want to write: if you want to be a researcher, you'd darn well better learn to write); a writer working towards a clinical degree might have the demands of a practicum or internship competing. And, of course, it goes without saying that there are always the rest of the demands of our lives separate from dissertation writing (or whatever other writing we might do).
If you are not a writer, and if you don't know how to make time, it's even more important that you try to use whatever time you can find--fifteen minutes here, twenty there. You have to be in the regular practice of writing in order to bring the project to completion.
It would be great to form a routine in which to write. I don't know many people whose schedules allow them to stake out a matter of hours at the same time every day. If you can't generate a routine, it's even more important to try to grab at the moments of time that present themselves. Every one of those stolen moments that you spend trying to write one paragraph about some subject, or one sentence for another--however you choose to spend the seized moment for writing--is a moment that you're getting yourself physiologically and psychologically accustomed to the task of writing.