Some applicants may ramble on about themselves in a manner that may appear self-indulgent and not very appealing to the committee. Remember, this is an application essay, not an autobiography. Conversely, some applicants tend to say too little, perhaps hesitating to promote themselves too explicitly or not knowing what about themselves would be interesting to people whom they don't know. In such cases, perhaps focusing more on what you want to do than on what you have already done (let your record speak for itself) may help in getting beyond self-inhibition.
Write at least three drafts of your essay. Think of your first draft as your rough draft. After you're done writing it, wait a day or two, then take a look at it with fresh eyes. Does your essay say everything you want it to? Try reading it from the perspective of an admissions officer or employer. Do you stand out? How can you rewrite your essay to make it better? After you're done writing a second draft, take another day or two, then look at it again. Are there any other improvements that you can make? Is this the absolute best essay that you can write?