A bunch of my friends and I maxed out English scores on various standardized tests while growing up (most of us without any kind of test prep class). I also taught English for the better part of a decade in my 20s. From everything I’ve seen, the single biggest factor is just reading. People who enjoy reading and who read one or more books per week pretty much always end up with large vocabularies. Reading older book or more niche genres probably helps, too, but I’m a bit skeptical of flashcards despite having gotten really into SRSes in the past.