In my experience kanji cards are the most effective way to learn kanji. The easiest way to get kanji flashcards is ordering ready made ones. Of course you can also use the free pdf files to print your own kanji flashcards. Finally if you need some more writing practice you can write your own cards.
Kanji cards from the book store
There is a single company dominating the market for ready made kanji flashcards. White Rabbit Press produces kanji flashcards sorted into three sets by difficulty and JLPT level.
|JLPT levels||N5 and N4||N3 and N2||N1|
Ready to print kanji cards
You can download pdf files containing ready to print kanji cards in the download section. They might not be perfect yet, but they get better with each update!
Howto Write your own Kanji Cards
Kanji cards could have approximately the size of a credit card or a business card. I use squared A8 index cards available from my local stationary store. When writing complex kanji the squares are more useful than lines would be.
Information on the front side:
Information on the back side: