i learn katakana too.. it needs less than a week for me to memorize it. i learn it by writing my diary in katakana. always. and it helps me a lot.
i also use java application on my mobile to take a quiz on my katakana+hiragana+kanji skill. or maybe if i forget how to write, i just check it on my mobile. so i can do it anytime, anywhere, without having to bring books of 'how to write kanji, haragana, and katakana' ^^