Invest in a set of quality flashcards that can be purchased at a book shop or online. In addition to having a set of Hiragana and Katakana flashcards, it also needs to have a breakdown of the Kanji characters and the order and direction in which the strokes need to be written. For instance, in the character for "tongue," the square at the bottom represents the "mouth." When you see the square character used in combination with other characters, it may be making a reference to the "mouth." Ideally, the flash cards also need to include sample sentences to demonstrate how to use the word.
Join a local Japanese-centric group or organization to practice your vocabulary. Conversing with native speakers also improves your pronunciation and introduces you to Japanese colloquial (slang) phrases or words.
Take advantage of Japanese-based media like TV shows or newspapers. Using this method, you can work at your own speed and focus your vocabulary lesson around topics of interest to you, such as fashion or politics. Many TV service providers have international stations available for an additional fee, so it's worth speaking with a representative to see what they have to offer in this arena. Local Japanese food markets also sell Japanese newspapers or magazines.
Consider purchasing an English-Japanese dictionary to carry with you. If you are new to the language, try one that shows you how to phonetically pronounce Japanese words. Otherwise, if you are familiar with Hiragana and Katakana, any English-Japanese dictionary will do. Practice speaking Japanese using phrases from your everyday life. For instance, if you go for a cup of coffee, practice ordering coffee in Japanese.
Visit the country. This is perhaps the best, and most expensive, way to learn or improve your Japanese vocabulary. Large cities such as Tokyo or Osaka often have English speakers to help with translation, so venture out of the big cities to truly immerse yourself in the language and the culture. Keep in mind that areas of Japan have their own dialect. For instance, the Kansai region near Osaka and Kyoto has a special regional vocabulary that you wouldn't necessarily hear in Tokyo. The same is true for Okinawa. But the overall language is similar and what works in one city works in another.