Read the brochure, check the website, gather all information possible to help you determine what the admissions committee is looking for from potential students. Your research should provide enough of a knowledge base about the school to tailor your essay to it. Show that you're interested and that you've taken the time to learn about the program. Take careful notes on each program and note where your personal interests, qualities, and accomplishments coincide. One size clearly does not fit all.
Many applications require that students address specific questions in their admissions essays.
Make sure that you're answering the question. Some applications offer a string of questions. Pay attention to your responses and try to avoid being redundant. As you begin write, remember that this is your chance to present your strengths and really shine. Take advantage of it.