When I am studying other languages, I think language games are really helpful! A friend recommended "Influent" (http://playinfluent.com/) or you can get some free apps or computer programs that let you play memory games. Just a little every day makes a big difference.

language is about practice. To memorize well, I usually write the word in a note and i will use it as often as possible. When I suddenly forget, i'll check it soon or ask the native speaker.