As soon as I acquired five languages, I realized that learning a language is not a difficult process. Let us considered following fact. To learn English it took me about ten years, then to learn Russian it was about four, then to learn Japanese it was less than three, finally however I am still learning Chinese took me not more than a year to speak fluent level. Obviously, this does not mean that I speak all of the languages as a native speaker, but I have enough knowledge to communicate and understand everyday conversation and maybe more advanced topics. So when I started to learn English in primary school, naturally since I was a kid did not really pay attention to the class. I learned a couple of words a week and then I forgot most of them the next. I was about 17 or 18 when I learn Russian so I put more effort in it that is why it took less. When you came to Japanese I decided by myself that I will go to language school and start learning. This was my own decision unlike the previous ones. As such I have spent many many hours studying and reviewing grammar, vocabulary and expressions. Finally studying Chinese seems to be natural process and something like a challenge to my brain. For me consciously learning a language means that you spend some time every day on learning and reviewing. I would like to emphasize here, that reviewing might be more important than studying. In Hungarian we have a common saying, that says "reviewing is the mother of knowledge".
If you forget things it is natural. You should not worry about it too much. Thing that you should worry about its review. So when you realize do you have forgotten a word, you should immediately review it. There are many studies showing that our brain forgets the newly acquired knowledge very quickly. Suppose you have learned 10 new words a day. Let us see how many you remembered the next day. I am pretty sure it will be less than eight. And let us see, without reviewing, how many of the words you remember two days later. It could not be more than five. The connection between the elapsed days and the words remembered can be plotted on a graph, this is called forgetting curve.
One of the most effective method that I like the best is what I would call actually usage. In other words when you learn a new word, you try to use it from that moment. By doing so you do not even have to review it. You will realize that if you have used it for at least three or four times he will never ever forget it. It is because you will associate with the three or four situations that you have used at. It is something like a "heureka" feeling. An "Oh I remember I have used it before" feeling.
When you learn vocabulary by only looking at a piece of paper with your handwriting on it, is really ineffective. It is because by doing so you are only repeating empty words, however it is so much better to make connections in your mind. So when you learn a new word it is the best to make an image that you will connect to it. Naturally when you tried to say over in a foreign language first your mind things of the image and then you start looking for the word. If you do not have this connection in your brain it will not know how to get to the word.