Free resource
Anki
My notes
Spaced repetition is already built in, and if you're on iPhone and can't use Pleco for free, this is the alternative I'd recommend without hesitation. The premade HSK decks — both old and new versions — mean you can get started without building from scratch if you don't want to. I still think making your own cards is worth the time, but Anki makes either approach viable.
Spaced repetition is the most efficient method for long-term vocabulary retention, and Anki implements it cleanly without getting in your way. I would create cards immediately when encountering a new word — definition on the back, the example sentence from where I found it, and any usage notes I wanted to keep. The premade HSK decks available online are good enough to start with if you want to get going quickly, but the act of making your own cards is itself a study session: I always preferred building from scratch for that reason.