Flashcards, and the Leitner system, is a fantastic tool for memorising and drilling facts. Anything that can be studied in a quick "question and answer" format can be turned into flashcards – things like short definitions, foreign vocabulary, scientific symbols, historical dates, traffic signs, world capitals and other fast facts.
A flashcard is a piece of card with a cue on the front, and the answer on the back. The cue can be a question, a single word or a picture. When you review your flashcards, you will take one card at a time, look at the cue and try to answer it as quickly as possible before checking the answer and moving on to the next card.
However, it is more convenient to have flashcards on your phone so that you don't carry all paper flashcards.