Bates College gives some very useful tips regarding the college application. Short, sweet, and very effective!

What's the purpose of this essay?
One purpose, obviously, is to give us a sample of your writing. Liberal arts colleges place a premium on strong writing skills. We look for a mastery of the mechanics of writing (grammar, syntax, and organization) as well as for fluency and originality. Your essay gives us a taste of the maturity of your thinking and writing, and of your readiness for a competitive liberal arts program.
A second purpose is to enable you to share something of yourself that may not be reflected in your academic record or in your recommendations.

Some Tips for Writing
1. Offer us some insight. This is the time to recount a powerful experience or significant relationship (such as tutoring a handicapped child or discovering a passion for medieval art) that has changed your perspective or challenged your beliefs. Instead of merely giving us a chronology of your bicycle trip through France, you might explain how your responses to the culture altered your perceptions of your own country and yourself.
One applicant shared his urban upbringing by taking us with him on a daybreak run through the city streets. Another sent a journal she kept while she was living as an exchange student with a Greek family. Yet another applicant wrote about how playing a varsity sport helped him appreciate the value of teamwork in an otherwise individually competitive high school environment.

2. Be careful of the obvious. For instance, "How my trip to France taught me independence" is a bit too easy. But, if reflective, anything - travel, a significant personal struggle, a family experience - can be an impressive subject.

3. Social and political topics should be tied to previous interests or experiences. An essay that ponders the effects of poverty as perceived while volunteering to build a house in Appalachia could work. An essay on devotion to environmentalism as an abstract idea carries little weight.

4. Demonstrate your intellectual interests. Consider writing about your response to works of a particular author, research in certain areas, or ways in which you as a student have reached beyond your curriculum. In fact, we encourage you to submit additional writing samples (perhaps a copy of a term or research paper, poems, or even an in-class essay) that reveal an ability to organize thoughts and defend ideas under the pressure of time.

5. Write and rewrite! The essay is the closest possible model to a principal form of college writing, the term paper, so treat it as an example of your college readiness.

6. Keep an eye on presentation. The essay should be neat, readable, handwritten or word processed.