Flashcards are cards with a question on one side and the answer on the other.
Flashcards can be used to memorize any topics, but in our case they can easily be used for vocabulary and grammar points. See Antimoon's page on flash cards for examples of interesting flash card types. You could also use pictures on the other side of the card to help you learn vocabulary, or a verb on one side and it's conjugation on the other.
In spite of the fact that nowadays flashcards can be easily inputed into our computers it is really advantageous to create your own cards. This way you not only get to practice your writing but you can always take your cards with you wherever you go and revise at any moment. If you do start making your own cards remember to write clearly and use different colors.
If however you are in favor of new techniques, here is some software which will come in handy:
- Mnemosyne: a free flashcard program available for many platforms.
- Anki: a free flashcard program with additional special features targeted for learners of Japanese and Mandarin.
Flash card software generally implements a Spaced Repetition System, which will schedule the cards for optimal memory retention.
See Wikipedia: Flashcard for information on Leitner System.