Not getting a makeup trial.
As you walk down the aisle toward your spouse-to-be, you don't want the first look at you to be of abject horror because you look like a caricature of yourself. At the very least, don't book a makeup artist without seeing a portfolio of his or her work! You didn't do that for your photographer, so why would you do that for something as important as your face? This goes for your bridesmaids, too!