TL;DR … Flash Cards, but with a calendar to back it up.
Essentially, long-term retention of knowledge has been shown to be more effective if repeat review is spaced out over time (as opposed to cramming). It turns out that you want to slowly lengthen the time between review to maximize recall.
For educational purposes, the spacing should stretch to weeks in order to be most effective in long-term recall.
Spaced repetition is a learning technique that adjusts your time between reviews for individual items so that your recall is tested at the most ideal time for your memory. The goal is show you a review just before you would forget it. — From: WaniKani: SRS: Spaced Repetition System
The actual timings of the system I used (via the application, WaniKani's SRS Stages):
- 4 hours
- 8 hours
- 1 day
- 2 days
- 4 days
- 2 weeks
- 1 month
- 4 months
There are tools, like Anki, to help you create your own flashcards. There are flashcard sources all over the internet designed to plug into the tools (e.g., search for "Anki Decks").
The technique can be used for lots of things, but is particularly good for fact-based knowledge. Some sweet spots:
- Learning a language
- Studying for medical or law exams
- Memorizing people's names and faces
- Geography
- Musical chords
Japanese (along with Chinese) are the only logographic writing systems still in active use in the world today. (Historically, there are heiroglyphs [Egyptian and Anatolian], cuneiform, Mayan glyphs.)
What this means in practice is that you cannot read standard texts (like newspapers, books, websites) without knowing thousands of characters. The Japanese Ministry of Education defines 2136 characters (Jōyō Kanji) that can be used in official publications and are expected to be taught to every student who completes high school. To be functionally literate in Chinese requires knowledge of upward of 4000 characters.
By the way, Chinese characters constitute the oldest continuously used system of writing in the world.
Note that ALL other languages with a writing system use a pronunciation-based alphabet that is usually no more than 50 characters (Khmer has the most at 74 letters).
Aside: Although Korean used to be written with Chinese characters, modern Korean is written using Hangul, which has 28 distinct "letters" than are combined into syllables that can be mistaken for characters, but which actually represent a clear pronunciation knowing only the 28 letters.
My journey into SRS was via a paid service called WaniKani. I've been using it since September 2017 to build up my knowledge of Japanese. I now know the vast majority of kanji required to read a book in Japanese (I've read 3 so far).