It's a funny thing: people often ask how I discipline myself to write. I can't begin to understand the question. For me, the discipline is turning off the computer and leaving my desk to do something else.
Nonfiction requires enormous discipline. You construct the terms of your story, and then you stick to them.
I write every moment that is humanly possible. I write every day and every night. The only discipline I lack is the discipline is to quit.