Here is a link to an article by Jay Mathews a Washington Post columnist on writing college application essays, Read http://bit.ly/9PGLr9
Here is an extract I liked.
Write about something you care about, some quirk or habit or interest that defines you in ways not obvious from the rest of your application. One of my children wrote about his Little League coaching. One described her talent for identifying a song on the radio from the first few notes. One explained why he loved Howard Stern.
They found ways to use these themes, even the odd ones, to reveal a personal value that was important to them and, hopefully, impressive to admissions officers. I advised them to take one more step, the only original suggestion you will find in this essay. Reveal an endearing flaw, I said, some bit of self-deprecation that will convince the college that you would be a pleasant person to have around.
Is the essay about your love of chess? Describe the day you set your high school team's record for being checkmated. Are you writing about your effort to ride every bike trail in the state? Say how you felt when you got hopelessly lost in the woods and had to be guided to safety by a passing Cub Scout troop.