<p><nitpick> Actually, a contraction would be "don't" instead of "do not". </nitpick></p>
<p>A college essay is the opposite of professional. Every other part of your application--grades, test scores, awards, and so on--shows that you're capable of doing work. The essay is your chance to show off your personality, interests, and thought process, and the most effective way to accomplish that is to use your own voice. If you tend to write very formally, contractions would probably seem awkward or forced and you'd be better off without them, but if (like most people's) your natural style is more casual, they can help your writing seem more genuine and flow more smoothly.</p>
<p>Just as an example, I'm going to post a paragraph from my Common App essay, with and without contractions. You can decide for yourself which you prefer. :p
[QUOTE=me, again]
Being the daughter of two diplomats means having to be cavalier about things other people take very seriously, like moving to a different continent with three months’ notice. You go along with things; that is the whole point. You keep your bags packed—you will learn the language when you get there—and if nothing else, you will have a story to tell. (“I lived in Moscow when the Soviet Union fell. A Russian activist set my house on fire—no, I am serious!”)
[/quote]
The style of my essay is informal, even conversational, and in this case not using contractions would seem contrived. Instead, I use diction (word choice - e.g. "cavalier") to alert the reader that I am in fact articulate and capable of more elevated writing, but made a conscious choice to use a more casual tone for this particular piece.</p>
<p>In short, I'm generally in favor of contractions, but it really depends on the context.</p>
<p>Edit: I should probably add that I haven't actually been accepted to college yet, so take my advice with a truckload of salt.</p>