Spaced repetition is an evidence-based learning technique that is usually performed with flashcards. Newly introduced and more difficult flashcards are shown more frequently, while older and less difficult flashcards are shown less frequently in order to exploit the psychological spacing effect.

from Wikipedia

A good primer on what spaced repetition is and how it works can be found here. It is an article by Andy Matuschak, who, along with Michael Nielsen, have written a great deal on this topic (and also have an experimental book on quantum computing learning, which leverages the technique of spaced repetition). As a platform for spaced repetition, I use the free application Anki, which also has a paid mobile counterpart. Synchronization among the two instances is handled by the software.

Michael writes, on his own essay on the matter, that "while Anki is an extremely simple program, it's possible to develop virtuoso skill using Anki, a skill aimed at understanding complex material in depth, not just memorizing simple facts."

One moment Anki is asking me a question about the temperature chicken should be cooked to. The next: a question about the JavaScript API. Is this mixing doing me any real good? I'm not sure. I have not, as yet, found any reason to use JavaScript to control the cooking of a chicken.

There's also a reference to Moonwalking with Einstein while addressing the topic of memory palaces and some other similar techniques, and later to How We Learn.