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.
One option is to organize your essay from general to particular . For example, if you were describing the new Big Tex at the State Fair of Texas, you might start out by describing the setting - the smell of the corn dogs frying, the crowds of people, the happy children dodging in and out of the midway games - then get into his grandiose size - after all, everything is bigger in Texas, right? After that, you describe the details: his new crisp, white, pearl snap shirt, his blue jeans tucked into cowboy boots, adorned up top with his signature belt buckle, and so on.