More and more, I'm starting to rely on Spaced-Repetition systems (SRS) for learning. That's WaniKani for Japanese Kanji and Vocab, Memrise for good courses they have (like the one on Morse Code) and Anki for everything else I want to remember, like Spanish Vocab, books I've read, Latin expressions or the Nato phonetic alphabet. Once I've mastered a course, it will also be migrated to Anki for maintenance.
At the moment, I'm only have thousands of cards, but I'm quite curious how long-term maintenance is going to work once I reach tens of thousands and more. For one, reaching a sustainable pace (say, 200 reviews a day) on a moderate set (10k cards, that's just the top 5000 words for a single language) will mean reviewing each card at most every 50 days.
I'm also trying to record random facts that I want to remember in Anki, essentially externalizing my memory a bit, but actually ensuring that everything that I want to remember longterm actually stays in my memory.
It will also be interesting to see if there's a limit to what I can remember. Is it 100k cards? A million? I will come back in a year and review my progress.
In the meantime, if you are curious about SRS, try Anki, and make sure to set up AnkiWeb. You can also use it to download decks.