When I was studying Japanese in Japan, people used to ask me what did I think the most important part of learning Japanese was? If I could only study one thing what would it be? My answer, vocabulary. The more Japanese words you know the better you will be able to understand the conversation. Even if you don’t understand the grammar, you can convey your general meaning if you know the right Japanese words. However, the way that you study Japanese words are just as important. There are two important methods to gaining fluency with Japanese words. The first is the way you study the words. The second is the way you practice them.
Now I mentioned above that a large vocabulary is probably the most important aspect to speaking the Japanese language. It is even better if you can study them in a way that allows you to learn grammatical structures at the same time. The way to do this is to study words in context. Doing so will help you learn how the word is used and also give you practice learning related words and different types of sentence structures. I also find that learning words in context help me remember them quicker because I am creating a small story in my head rather than just trying to memorize.
The second part of learning words is to use them again and again. In college I would spend a night cramming to learn all the words before a test to get a high score. Once the test was over and so the practice also stopped, then the words were forgotten. In order to fully lock Japanese words into your memory, you need to use them over and over again. This means that you will need to continuously review new material until you know it very well. A program like Mnemosyne, a flash card program that tracks how well you remember words can be very helpful for this. Of course the best way to do review words is to use them in conversation. This will allow you to put them into context and to make sure you can both hear them and speak them.