Does everyone know about #Anki? Spaced repetition is the idea that we forget things slower and slower the more we remember them. We can use software to keep track and only remind us when we're about to forget. (Like Duolingo, if you use that crap.)
https://apps.ankiweb.net/
It's actually a free software clone of SuperMemo, whose original creator basically did a ton of research on learning and came up with this brilliant tool. Now Anki is even better than its parent.