If you, however, want to keep words long term, you need to revise them in the course of a few days repeatedly using the so-called space repetition.
You can use apps which are based on this system such as Anki or Memrise, or you can write lists of word in a notebook using the Goldlist method, which is also very popular with many polyglots.
If you're not sure which methods are effective and what is available out there, just check out polyglots' YouTube channels and websites and get inspiration from them.
If it works for them, it will most probably work for you too.
The third principle to follow is to create a system in your learning.
We're all very busy and no one really has time to learn a language today.
But we can create that time if we just plan a bit ahead.
Can you wake up 15 minutes earlier than you normally do?
That would be the perfect time to revise some vocabulary.
Can you listen to a podcast on your way to work while driving?