High GPA's, SAT scores, and myriad extracurricular interests are usually not enough to distinguish one good candidate from another. The three components of the application process that allow a glimpse of each applicant as a unique person are written recommendations, the interview, and the essay.
Suggestions concerning recommendations and interviews are available elsewhere.
As a widely published essayist I offer advice here . Beyond grammatical correctness lies forceful, vivid prose. A good edit involves the reshaping and rephrasing of ideas to lift the writing to a level that will capture the attention of admissions officers. This is probably the most important 500 words you will ever write. If you have questions or comments, you email them to me.