Of course, the best way to improve your vocabulary is through practice~! I heard that to really remember a new word you need to hear it 7 times! So it is important to constantly practice and be in contact with English.
You can do this by reading in English. If you think a book is too difficult, try a magazine or Penguin's Graded Readers. To know your level, read one page and if you don't know 1-3 words then it is your level! If you don't know 5 or more words on one page then it is too difficult and you will not enjoy reading. The great thing about the graded readers is they will help you boost your vocabulary and also improve your reading speed.
Videos are also really good, thanks to DVDs you can practice your English listening and learn new words. The video helps you to easily understand what they are talking about. To use DVDs effectively try this:
- Watch the DVD in Japanese.
- Watch the DVD in Japanese with English subtitles.
- Watch the DVD in English with Japanese subtitles.
- Watch it in English with English subtitles.
- Watch it in English without subtitles.
So, you will be watching the DVD often to practice your English so it is important to chose a video you really love.
The internet also gives many websites that can help. Here are two Japanese-English flashcard sites. They are made for English speakers learning Japanese but should be useful for you too.
http://www.schei.com/flashcards.htm
Also, try quizlet.com to make your own flashcards!
You can also use various word of the day websites:
Oxford's English Dictionary's
http://www.oed.com/cgi/display/wotd
Merriam-Webster's
http://www.merriam-webster.com/cgi-bin/mwwod.pl
For English as a Second Language learners:
http://www.eslmonkeys.com/student/wordoftheday.php
And you can sign up to have a new word emailed to you~!
http://www.eslmonkeys.com/student/wordoftheday.php
I hope that helps~!
Phil