Writing well on literature and the arts
-Look very closely at the language/image.  “God is in the details.”
-Make it precise.  Make precise the thought in your expression of it in words.  (Thought and expression are essentially the same).

Writing well in general
-Use an outline if it helps you.  If so, each point should be a sentence (the minimal unit of a thought).  The outline has a tree structure; at the top is a single statement that gives the claim the paper will argue for (the “thesis statement”).  Do not confuse this with a phrase that says what you are writing about.  You must not only write about something, you must say something specific about it; this must be a claim that is controversial (not obviously true), that would make a difference if it were true (if you succeed in showing it to be), and this will be what the paper as a whole tries to prove.
-Revise.  Reread your paper asking yourself if it is clear, if you prove your point, and if you said precisely what you want to say.  Do this as many times as needed and as you have time for.  (This is not a substitute for having an editor, who will notice problems in these areas and others if they are there).