Also, I would higly recommend experimental poet Kenneth Goldsmith's [1] book "Uncreative Writing: Managing Language in the Digital Age" for those struggling with writer's block. In the context of this discussion one could say that his entire art deals with issues of writer's block very closely (e.g. he has "Soliloquy", a 800-page book consisting of every word he spoke during a week, etc).
A passage from "Uncreative Writing":
"There’s one solution that each and every book on writer’s block offers: write five words. Any five words. Follow this advice, Mr. Ashbery, and you’ll never have writer’s block again."
[1] https://en.wikipedia.org/wiki/Kenneth_Goldsmith; see also Ubuweb, the massive avant-gart art catalogue he curates, at http://www.ubu.com.
Oh, that's easy: tests, documentation, ... :)
Seriously though, one way to stop on a roll is simply not to commit the code into the repository, even though it appears good to go. The next day there is a git diff waiting for you. You now have to write a bit of prose in the commit comment to explain it, and that can get the ball rolling.
Do: Always stop working a bit early while you're on a roll (in the zone), for ex: while you still know what to write next. Then the next day, or next writing spurt, you know exactly where to start and get back into the flow of writing. The juices will flow easier.
Don't: Push yourself too hard during moments of productivity, for ex: staying up late pumping out everything you have in your head until you can't write anymore. The next day you'll be starting from zero and more liable to find a hundred distractions before starting 'real work'.
If you're scared of forgetting something in your head write some summary notes after you stop, then use them the next day.