Here is a final checklist of things you should do and should avoid while writing your essay:

Get personal.
Be yourself.
Tell a story.
Use details, but not excessively so.
Be honest and sincere.
Write about what interests you.
Plan before you write.
Show instead of tell.
Be specific.
Start writing your first draft early.
Be concise.
Establish your identity and write in the first person.
Elaborate on experiences that demonstrate your personality traits and motivation to pursue medicine.
Turn weaknesses into strengths. Steer the essay toward your strong points.
Be self-confident.
Have Leslie North look at it as soon as possible so she can tell if you're going in the right direction.
Schedule a Writing Center conference.
Revise, proofread, and read aloud.
Pay attention to the character limit.

Mention high school accomplishments unless outstandingly relevant.
Discuss controversial subjects like politics or religion or something from your past that could be embarrassing.
Be overly creative or use gimmicks; you may come off as cutesy or immature. Err on the conservative side.
Write what you think the admissions committee wants to hear.
Be overly formal.
Exaggerate or embellish.
Use jargon or technical language.
Write about what it takes to be a good doctor (your audience already knows this) or preach to the committee.
Use a thesaurus if you can avoid it. If you do use a thesaurus, only use words that you know the exact meaning and connotations of (a dictionary can help you with this).
Name drop.
Begin every sentence with "I."
Start with a joke unless you're naturally funny.
Underestimate the importance of the essay.