How do you develop the discipline to write?

Dear Literary Ladies,
Some days, I just can’t find the resolve to work. I could blame all sorts of distractions and interruptions, but maybe it’s the discipline I lack. If words don’t flow right away, I’ll get up and find some fine excuse not to stick with it. How did you develop the discipline to sit down and just write?

Ultimately, you have to sit down and start to write. And even if all you do is type out “I can’t write this morning; I can’t write this morning; oh, bother, I can’t write this morning,” that will sometimes prime the pump and get it started. It is a matter of discipline. It is particularly a matter of discipline for a woman who has children or another job.

I think my years in the English boarding school where I had to create my own privacy were also a way of learning to create my own discipline. Now there are mornings when I joyfully sit down at the typewriter. But there are mornings when it is anything but a joy. There are evenings when I go to the piano and the music comes pouring from my fingers. There are evenings when I’m all thumbs and I have to make myself sit there and go over scales and finger exercises before I can play anything. The same thing is true with writing.

—Madeleine L’Engle, from Madeleine L’Engle Herself, 2001