David Epstein’s book Range educated me on the value of interleaving and spacing for better learning. (Chapter: Learning fast and slow)
One of the places I immediately applied it is in my daily Spanish lessons on Duolingo.
Previously I used to start with one skill in Duolingo, say present perfect, and then complete it from start to finish. I only moved to the next skill once the previous skill was golden, or on the rare occasion when I gave up on it for being too hard.
The screen looked like the one on the left: all golds above the current skill.
Now I have six skills in progress at the same time. Every day I complete just one test from at least three of them. The next day I start with the other three. If I want to practice more, I use the dumbbell button in the bottom right—it tests me randomly from any of the dozens of skills I have already completed.
This mixing provides me with a bit of range. Each test daily is from a different skill; any skill reappears only after 48 hours; forcing me to remember, forcing more mistakes, and, hopefully, resulting in better learning.