Nothing
The important thing is that there should be a space of time, say four hours a day at the least, when a professional writer doesn’t do anything but write. He doesn’t have to write, and if he doesn’t feel like it, he shouldn’t try. He can look out the window or stand on his head or writhe on the floor. But he is not to do any other positive thing, not read, write letters, glance at magazines, or write checks. Either write or nothing … I find it works. Two very simple rules, a: you don’t have to write. b: you can’t do anything else. The rest comes of itself.