Using words that you learn is the best way to remember them. So reading, writing and saying the new words will encourage long-term memory. Learning words that are relevant to what you want to say will also help. If you have not tried a spaced repetition system (SRS), such as Anki, take a look. SRS systems work by testing you at appropriate intervals, encouraging you to review words at about the time you are likely to forget them. They work on the principle that initially you need to see/hear a word often but as you become more familiar with the word, you need to see/hear it less often. Hope this helps.

I think there are 3 main steps to help with this problem The first one is to get a vocabulary app. Flashcards deluxe app is a really good app for familiarising yourself with new vocabulary. Just using it for a few minutes per day can really help internalise new words. http://flashcardsdeluxe.com/flashcards/ Then reading a lot in English will also help, because you will see these words in context. Through reading, you will be internalising your new vocabulary without realising it. The, the more practice you get with speaking in English, the easier it will be to start using the vocabulary and to stop substituting it for easier words.