Forgetting kanji, that is. I knew some kanji when I went to Japan, but I came back knowing many more, most of which I promptly forgot upon returning to school. I can recognize the bulk of the kanji that I've learned in the past, but writing it is another matter. Sometimes I forget how to write words that I learned my first year of study. This seems to be a common problem for most learners (and native-speakers) of Japanese. Once I was complaining about not being able to remember how to write something, and I was told "It's OK, Japanese people forget that kanji too."
While I have to admit that did, in some perverse way, raise my self-confidence, that kind of rationalization ultimately is of no use to anyone. Especially if that someone is studying for the JLPT - Japanese Language Placement Test. Yes, I am currently trying to bring my kanji knowledge (and my grammar) up to par for the JLPT. The first test is coming up in July, but I am signing up for the one in December. This gives me about 4 months to double the number of kanji I can read, learn some (mostly literary) grammar forms, and extensify my vocabulary (which is apparently small enough even in English that I feel the need to create words such as 'extensify').
There are many sites available for the purpose of studying kanji, but I've been using one called JLPT Kanji Project, which has flashcards separated by JLPT level. The design is easy to use and easy on the eyes. If you sign up for an account, you can put kanji you don't recognize or that you'd like to study further into a separate folder (up to 250 at a time). They also have a vocabulary section - in fact, when you click on a kanji to see its meaning and pronunciation, the description also includes relevant vocabulary, ordered by JLPT level. For example, the description for the kanji 艦 has a breakdown of its meaning (warship), pronunciation (かん), and the Level 1 vocabulary in which it appears: 軍艦.
Hopefully I can get my kanji knowledge up to the standard 2,000 or so. Once that struggle is over, the struggle to maintain it begins.