Whatever you are writing, whether it be an essay, newspaper article or book it is generally rule of thumb that you know what the heck you are writing about. Do your homework on the subject, research it to make sure that you really do know what you are writing about. If you don't know the subject well it will reflect in your writing.
Another rule of thumb is don't try to impress the reader by using big, uncommonly used words which some of us have probably never even heard of anyway. There's nothing worse than having to read a book with a dictionary by your side, it's frustrating and off putting.