I agree 100% with STEM2017. Read the “Little Book,” cover to cover. Twice. Three times. Take the time to digest it.
Especially take to heart Rule No. 1: “Omit Needless Words.” Learn how to apply the rule to your own essays.
“Vigorous writing is concise. A sentence should contain no unnecessary words, a paragraph no unnecessary sentences, for the same reason that a drawing should have no unnecessary lines and a machine no unnecessary parts. This requires not that the writer make all his sentences short, or that he avoid all detail and treat his subjects only in outline, but that every word tell.”
I first read this book in my freshman year of college. Sometimes after writing a first draft of an essay, I would pick up my copy of Strunk and White and re-read the whole book (it’s not very long). That would put me in a frame of mind to “swiften up” my essay. I proceeded from the assumption that it is ALWAYS possible to cut 10% of my draft essay. I could sometimes apply that principle 2 or 3 times over to the same essay until I’d omitted enough needless words.