One of numerous ways to overcome the problem of forgetting words is to write them out. You can do this while you are learning a word to remember it later on. Or you can write a whole sentence out as a punishment during your reviews. I did the following to try not to forget words.
Copying the words into a browser window, and hover over it with Rikaisama set to Japanese.
Messing the desktop with notes containing the words and the English translation.
Write out the words during reviews.
Write the word numerous times in Kanji and Kana.
Write the whole sentence.
This was not working very well and it was time consuming. So I had to come up with something better. And here is how I did it. For sentences containing words that I haven’t learned yet, I started looking up Japanese definitions, and adding them to my Notes field on my cards. I always have those definitions on the front, because the idea here is, to be able to see the reading of the word. I made it a habit to look up the Japanese definition first. In case that the definition is too complex, then see how complex it is, I simply add the definition in English. If not, I use the definition, and look up the words for that definition, and add them to my cards as well. If a definition contains too many unknown words, I limit the amount of definitions in Japanese, and add English definitions for everything below the first Japanese one.
Now, when I have a sentence that contains a word that I should know because I learned it, I do the same thing. I look up the Japanese definition first, no matter how complex it is, and add it to the front of my card. If you want to fine tune it, you can use a Japanese definition for the word, add all subsequent information for words that you don’t know in English or Japenese, and then guess the reading when the card is due. Here is an example card to better understand how this looks like.
The definition on the front is not there for you to cheat. Because if you wanted to do that, you can simply add the word with furigana to the front. The reason for having it all there is, that you have to actively look up and add information in Japanese and English, that does the trick. I found that when cards containing words that I have or haven’t learned come up next time, I don’t even have to look at all the definitions, to remember the reading and meaning. It greatly speeds up reviews, and if I need it again, I can just look at the reading, and all the other information.