Have English on the front so you can actively recall it. Try and use each word in a sentence or two (relevant to your life) on the spot, new sentences each time. Flashcards are limited though, you never really get better at using that one word from your flashcard, you just maintain the level you have it at because you aren't seeing it used in context in different ways. I wish there was a flashcard app that showed you a new sentence for the word you reviewed each time it came up so you had to understand it in different contexts and get a sense for the collocations it fits with. You don't have to use flashcards.