Yeah, it's mostly about reading I think. Just read read read really good quality A+ essays, and note not only the sophisticated vocabulary/sentence structure, but also the fluency - they don't jam in "sophisticated" words everywhere, that generally tends to be overwhelming and tiresome.
Notice the way often simple and direct sentence structure is punctuated occasionally with a "bling word", as my lit teacher calls them, so it seems natural and just a part of the student's vocabulary much as any other "simple" word.
I think the best way to develop an idea of what is fluent and what is too much is by reading high quality essays and also proofreading your own and editing like its your job! (Even do this after they've been marked etc if you're practising timed conditions. It's always helpful even if it's a "finished" essay.)
Happy studying!