I'm actually studying Japanese. Or, well, was. My problem is that I'm hopelessly lazy and when I was actually studying, it was kind of... All over the place. I know Katakana, Hiragana, quite a few kanji, along with a few random words. On top of that, I know a small amount of the grammar, but I really need to look over it some more.
So what probably caused me to stop studying? I never really had a schedule or anything for it, I'd just study when I remembered, which slowed it down while also keeping me from thinking "Oh hey, it's XX-o'clock! I should study!", on top of that, the fact that I didn't practice often enough or write much down, including the stuff I had problems with. If I had any tips, it'd probably be to take notes of some kind, it'd help you if you just need to look up something without having to go through an entire book or finding it in a program or searching for it on the web. Also patience. Dear lord, patience. Impatience is part of what caused my problems.
I actually might wind up giving it another go soon, only this time I'd prepare a little more beforehand.