The method that has worked for me, in order to learn new words, is reading as much as I can in the language I intend to master. I have started with books with basic vocabulary, according to level (Level 1, 300 words, level 2, 600 words, level 3, 1000 words, level 4, 1500 words, level 5, 2000, 6, 2500.) There are many series of graded books for readers students of English as a foreign language. Another way that has helped me a lot is listening to radio podcasts. When possible, also reading and listening at the same time. Reading and listening creates an environment similar to the one that have native learners. The most important words come again and again. Doing that we memorize naturally. If you are an advanced student, You can read also newspapers in the internet. In The New York times, for example, if you do not understand a word you can just click and get the meaning instantly.

It is only one way to make them stuck in your mind. You should use them in your everyday activities, or if it is some academic vocabulary you should use it in essays, or even in the italki notebook entries. It will help you to keep them in your mind. Good luck!

I try using them as much as I can so I won't forget. I know it's hard, but we just have to keep practicing. :) Making flash cards helps as well.