Capture your reader's attention. The college admissions officer will probably only spend a few minutes going over each essay. Don't spend time giving background information, just dive into the meat of your essay. Sometimes it is easier to write a draft of the essay and then come back and cut out the first few sentences completely.
Write about something important to you that allows you to answer the essay question completely. For example, if the essay question is, "Tell me more about yourself," rather than write several paragraphs detailing your quirks and habits, reflect on a significant event in your life and describe how it shaped you.
Revise your essay carefully. Check for spelling and grammar errors and look for areas of weak writing. Cut general statements that can apply to anyone, sentences that don't directly answer the question and controversial opinions unrelated to the topic from your essay.
Ask for a critique. Have several people, such as parents and teachers, read your essay. Ask them to write any notes or points that need addressed on a separate sheet of paper. Sit down with your essay and the critique papers and revise as necessary. Take others' opinions into consideration, but don't feel like you need to implement every suggestion. Don't allow others to edit your paper so heavily that it loses your voice or sounds stilted.