Write it down!
I can’t count how many times I have ‘learned’ a new word, and then promptly forgotten it again. I ask someone how to say something, they tell me, I repeat it back, and half an hour later, I have no idea what the word was.
It doesn’t really help that I am learning Chinese at the moment, so it’s more difficult to put the sounds, pinyin (Romanised pronunciation), and character together than it is for languages that use a similar writing system to my native language.
Nevertheless, I have found that, unsurprisingly, writing these words down helps. I managed to find a perfect little vocabulary notebook for this purpose (it has columns for word, part of speech, pronunciation, and meaning), though of course any notebook could be used for this. And in this technologically-driven world, a mobile phone, organiser, or even music player could do the same job.
Making a note of the word will reinforce it in your mind, and if you reorganise these words alphabetically or by group, and periodically read back through your collected words, they will become more familiar much faster. Not only will you be reminded about these words more often, it is likely that the words and phrases you discover on your own (rather than from textbooks or classroom material) will be more relevant or useful to you.
I’d better just make a note of the word for eggplant, before I completely forget it…