There’s an entire section on this forum devoted to the topic of College Essays:
http://talk.collegeconfidential.com/college-essays/
I agree with post #9 - I asked a friend who had worked in college admissions who had never met my S to read his draft. He made a few very small suggestions and made a good essay even better. My S’s main essay for the Common App wasn’t about anything huge and momentous, but did a good job at conveying his personality and his sense of humor.