This is all really good advice, so I'll only add a few other points.
- FOLLOW DIRECTIONS!! If it says "one page," don't go beyond that. If it says "type," then type. I can't believe the number of students who either don't pay attention or somehow think the rules don't apply to them. It's not an auspicious beginning when you don't follow the guidelines you've been given.
- MAKE SURE IT'S CORRECT!! Again, I can't tell you how many sloppy essays and answers to questions I've seen over the years. Your writing is representing you, so get rid of any "spots and wrinkles" you can before submitting your work. You wouldn't go to a job interview with stains on your shirt or holes in your shoes, so get rid of all the careless errors to make yourself more presentable.
- DON'T JUST REPEAT what's already in the rest of your application. That's wasting an opportunity you have to let them know who you are rather than what you've done. This is especially true if you have a less-than-perfect track record in one area or another. Use this opportunity to tell them what makes you someone they could see as a valuable asset to their school.
I believe that the most important thing to keep in mind is that you want your essay to stand out and be noticed. Colleges and universities will be receiving numerous applications and many of them will be the same. You most certainly do not want yours to fall in with the others. You will want your essay to be remembered. Also, make sure that it is completely free of errors. Proof read it again and again and have other people proof read it as well.
I would definately talk to your high school english instructor. He or she will have great advise for you and will be able to help you as well.
state the truth and only the truth
Since the deadline is approaching for early action and decision, and regular admissions is December 31, I have three general rules for my students: Be Bold, Be adventurous, Be courageous. This is the time to tell the college who you really are, and it is not the place to give them a rehashing of what is on your resume or in your aqpplication already.
Give them something that is from the heart, well thought out, and honest, and you should do OK. I would not go through any book of students who were admitted to the Ivy league as it does not fully represent the candidate and can not possibly compare to your own unique situation.
Best of luck where you want to apply!