For Japanese, If u'd get a habit efficiently to keep English, i recommend a RADIO-ENGLISH "徹底トレーニング英会話". The habit using BOOK and CD or RADIO is very cool. Cuz, there's ready to keep reading, writing, speaking and listening to english.
For instance, expending 3days in one week, We can get an episode which will make a good theme for studying. At the same time, there is some practices to dictate. On Thursday, we can get more practices about the episode. On Fri, we can get, furthermore, some short episodes to use vocabulary. On Saturday, more dictations and listening test are there.
if u haven't had this habit, would u try it? i believe it help ur english studying.