In an October 1935 article in Esquire. Hemingway offers this advice to a young writer:

The best way is always to stop when you are going good and when you know what will happen next. If you do that every day when you are writing a novel you will never be stuck. That is the most valuable thing I can tell you so try to remember it.

Reformulated for programmers and equally valuable:

The best way is aways to stop when you are going good and you have just written a failing test. If you do that every day when you are writing a program you will never be stuck. That is the most valuable thing I can tell you so try to remember it.

If you start programming for the day, a failing test to fix will get you right back on track.

Instead of having to muster willpower to get started and brainpower to figure out what you were doing and what to do next, you can mindlessly do whatever it is that will fix the test.

After that, you'll be much more likely to be in the flow of things, and be able to keep going in good spirits.