In addition to the good advice posted above me, make sure your statement is NOT a reiteration of your resume / ECs. Choose a few that come together to form a theme...I think most good statements have some type of theme to them. Schools read like 10,000 of these, so make it a good read. Some examples of friends' themes/topics:
*taking growth hormones throughout life
*being a music major and its connection to medicine
*relationship with little brother (big brother program, not a biological brother)
*being a Spanish major and communicating w/ patients (that was mine)
*Discuss a significant experience travelling
Your statement should make you stand out. If there was an event/person in your life that inspired you, that would also be good.
Don't sound too cocky when you write, but you should sound confident about your life experiences and your potential to become a doctor
Then have a ton of people read and critique your statement, especially your premed advisor if still in school. My school also has a writing center that was very helpful.
Got that? Now get to work! :wink: Good luck!