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.