Avoid the List
Many college applicants make the mistake of trying to include all of their accomplishments and activities in their application essays. Such essays read like what they are: tedious lists. Other parts of the application provide plenty of space for you to list extracurricular activities, so save your lists for the places where they belong.
The most engaging and compelling essays tell a story and have a clear focus. Through carefully chosen detail, your writing should reveal your passions and expose your personality. A thoughtful and detailed narration of a difficult time in your life tells far more about you than a list of competitions won and honors achieved. Your grades and scores show that you’re smart. Use your essay to show that you’re thoughtful and mature, that your personality has depth.
A Touch of Humor (but just a touch)
While it's important to be thoughtful and mature, you don't want your college application essay to be too heavy. Try to lighten up the essay with a clever metaphor, a well-placed witticism, or a little self-deprecating humor. But don't overdo it. The essay that is filled with bad puns or off-color jokes will often end up in the rejection pile. Also, humor isn't a substitute for substance. Your primary task is to answer the essay prompt thoughtfully; the smile you bring to your reader's lips is just a bonus (and a tear can sometimes be effective too). Many students have been rejected for failing to take the prompt seriously and writing essays that end up being more foolish than clever.
Tone, Tone, Tone
Not just humor, but the overall tone of your application essay is remarkably important. It's also difficult to get right. When you are asked to write about your accomplishments, those 750 words on how great you are can make you sound like a braggart. Be careful to balance your pride in your achievements with humility and generosity towards others. You also want to avoid sounding like a whiner -- use your essay to show off your skills, not to explain the injustices that lead to your low math score or failure to graduate #1 in your class.
Reveal Your Character
Along with the essay, most colleges rate "character and personal qualities" as extremely important in their admissions decisions. Your character shows up in three places on the application: the interview (if you have one), your involvement in extracurricular activities, and your essay. Of the three, the essay is the most immediate and illuminating to the admissions folks as they read through thousands of applications. Remember, colleges aren’t looking solely for straight "A"s and high SAT scores. They are looking for good citizens for their campus communities.
Grammatical problems, punctuation errors, and spelling mistakes can hurt your chance of being accepted. When excessive, these errors are distracting and make your application essay difficult to understand. Even a few errors, however, can be a strike against you. They show a lack of care and quality control in your written work, and your success in college partly depends upon strong writing skills.
If English isn't your greatest strength, seek help. Ask a favorite teacher to go over the essay with you, or find a friend with strong editorial skills. If you can't find expert help, there are many on-line essay services that can provide a careful critique of your writing.