I know I've said it before, but there's nothing like a deadline.
Fake deadlines, real deadlines, as long as there's someone you're letting down if you don't make it, or as long as there's a definite end point beyond which you can't continue, it will work.
Par example, each day I know I have to get out of the library at a certain time. When the announcement comes over the loudspeaker saying that we need to get out because they're closing, I know there's half an hour left. I reckon my best work is thanks to that guy. If that guy could make threatening announcements all day, I'd be as prolific as Bryce Courtney. And possibly as unrelaxed.
Another very real deadline: the AC power on my laptop isn't working for some reason. I have forty minutes left before it goes to sleep.
So, you'll have to excuse me while I write a novel. A short one.