In my opinion, Spaced Repetition Systems (SRS) are the best way to retain vocabulary - provided you use a good deck (the vocabulary ordering should make sense a

In my opinion, Spaced Repetition Systems (SRS) are the best way to retain vocabulary - provided you use a good deck (the vocabulary ordering should make sense and, IMO, example sentences are key). I use Anki but there are many alternatives. Otherwise, immersion through Spanish media helps - you can listen to Spanish music or watch movies/series in Spanish (initially with English subtitles, maybe later with Spanish ones).