There are a number of good resources on the web to assist you in writing an outstanding college essay. Several links are listed below. The essay should be consistent with the information provided in other parts of your college application, but it shouldn't be repetitive. It should support and expand information provided elsewhere. For example, if your grades and test scores are outstanding, you don't need to state that you're an excellent student. You may, however, want to discuss what motivates you or how you've developed your work ethic and how it applies to other aspects of your life.

Colleges also want to see a sample of your writing, so the essay should be well-crafted and completely free of errors. Be sure to brainstorm ideas, pre-write and write multiple drafts. Enlist others to proofread your essay and offer suggestions.

Finally, your essay should clearly show how you're an excellent fit for the institution to which you're applying. Study the philosophy and mission statement of the school and find ways to speak to how your interests, abilities and accomplishments reflect the school's philosophy. This means you can't simply recycle essays. You might be able to use ideas from previous essays, but each essay should be unique and tailored to the school to which you're applying.

http://collegeapps.about.com/od/essays/a/essay_tips.htm
http://www.quintcareers.com/college_application_essay.html
http://www.collegeboard.com/student/apply/essay-skills/  (or simply visit collegeboard.com and search for information on college applications essays)