Read the question or group of questions multiple times. Make sure you fully understand and address each part of the question. If a question asks what and why, don't forget to fully explain your reasoning.
Research about the program, faculty and school itself. Address what aspects of the field you are interested in and why. Specify what aspects of this school in particular appeal to your interests and why you chose this school over any of the other thousands in the nation.
Look at yourself. Relate the field you aspire to enter to your life and how your experiences tie into it. Bring special skills to the attention of the admissions committee. If you really cannot think of something, ask your friends and family to describe you. If they choose a word such as dedicated, ask them to explain exactly why and where you showed how you are a committed and responsible individual. Use specific examples.
Write to the requirements. If you are required to write one page, do not write two sentences. Do not go over the limits either. Longer is not always better so do not write a three page essay when the topic is a one page essay.
Have at least two people read your essay and critique it. Tell them to be harsh and give as much criticism as they feel necessary. Make the revisions they ask for and do not allow simple grammatical errors to go unnoticed.