When it comes to reaching language proficiency, vocabulary is
important. To reach proficiency as quickly as possible, the choice of
vocabulary is important.

I'm going to explain my approach a bit.

How To Learn Vocabulary

Vocabulary from a spoken standpoint requires quick recall without a
reference, which is really only attained by practicing the same exact
things over and over again.

I use flash cards, because they're the most
straightforward review method. I have two flash cards per word, one
testing translating from the language, and another translating to.

Anki is a great choice for any flash card learning, because it stores
your data about what you've learned, and modifies the frequency of the
cards accordingly: http://ankisrs.net/

Regardless of the medium, the real challenge is investing the time and
effort. It's very easy to stop once you're a week in, or a month
in. In order to minimize this risk, vocabulary needs to be a routine:
it must be a part of one's daily schedule.

For me, I've made my commute to and from work into review time for my
language flash cards. I listen to the same playlist (something with no
vocals), start up Anki, and get to work until I've reviewed them
all. I've been doing it with German for over a year (roughly 30
minutes per weekday), and I've learned about 800 words completely.

Per word, it's taken me 7500 (~50 weeks * 5 days * 30 minutes a day) /
800 ~= 10 minutes a word. Not bad (although some are easier than

What Vocabulary to Learn

Timothy Ferris has a great blog post about
learning languages (a couple,
actually) where he talks about finding the most frequently used words
and learning those first. It's a statistical method that is the best value for
your time in terms of vocabulary.

For any given language, a Google search will probably find a list
somewhere. In the case of Japanese In the case of Japanese, there's a
great post showing the most common terms used:
http://pomax.nihongoresources.com/index.php?entry=1222520260. Even if
you don't understand the grammar at all, the vocabulary alone helps
you get a great context on a conversation.

It's worth emphasizing the distinction between spokend and written
word frequency. My suggestion would be start with spoken.

My suggestion would be to learn the first 100 or 200 words. This
provides a solid base of roughly one half of all spoken words.

Where to Find More Vocab

After you have a good base set of words, I would move on to learning
vocab that really interests you. Find a book, tv show, or game that is
available in your target language, and add any vocab you don't know
into the flash cards as you go.

For me, I wanted to learn German. Luckily, I love video games, a
medium that fairly often has spoken dialog and text. I found a game
I've wanted to play for a while, converted all text and audio to
German, and started it up.

As soon as I encountered a word I didn't know, I paused the game,
wrote up a flash card, and continued to play. As soon as I reached
100 new words, I stopped playing the game; I was only allowed to
resume when I had finished learning all my outstanding vocabulary.

The beginning of this process was incredibly painful: I would play for
maybe half an hour before I had over one hundred words to cover! But
as time passed, I learned more, and the game began to move a bit more
quickly. I also sometimes found places in the game where I was unable
to pause. In that case, I used youtube videos of others playing the
game to supplement: let's plays in German

For Japanese, if you like video games you're there: there are lots of

video games with full audio.

If you prefer TV or movies, I'd suggest a website like Crunchyroll or Netflix. I think
Japanese comics (Manga) is also a very good choice: unfortunately it's
not available readily online. However, I've found that buying whole
series' from EBay to be a straightforward process.