Take time to think about and write your essays. Try to start before the fall of your senior year.
Make sure to actually answer the essay question.
Write about something that you care about.
Read through applications from the schools you are applying to for any repeated topics. You might want to choose those that are and minimize the number of essays that you write.
Be honest and genuine in your essay - when you're faking something, it comes through.
If you're writing about a certain experience or person, focus on how this experience or person has impacted YOUR life in a significant way. Your essay is about YOU, not someone else.
Read your essay as if you were the admissions officer and ask what it tells about you as a person. Does your personality and character come through?
Sound like a person when you write. Big words and overly formal sentences can make your essay sound stiff and forced.
Have someone you trust, like a teacher, guidance counselor, or parent read your essay and try to listen to his or her feedback.
Proofread, proofread, proofread, and when you think it's perfect, proofread again. A small spelling mistake can cost you.
Don't wait until the last minute to start working on your essays - when you're rushed you can't think as well and risk making careless errors.
Don't be intimidated by the task of writing essays - it's not easy, but you can do it!
Don't use cliches - they take aways from the genuine and honest nature of your essay.
Don't exaggerate - you should play up your achievements but don't go overboard.
Don't recycle essays "blind." You can use the same essay for two difference college if their essay questions are the same or very similar, but recycling essays without thinking can cost you the chance of admission.
Don't try to write what the admissions officers want to hear - be genuine and be you.
Don't overuse big words to sound impressive.
Don't whine - when writing about a difficult experience, focus on what you've learned from it and how it has made you stronger/smarter/more interesting as a person.
Don't make your first draft your last - you can always improve and it's worth the extra work.
Don't rely on your computer's spell checker - it misses tons of mistakes. Go over your essay carefully with your own eye and ask your parents or teachers for help in proofreading it.