One thing that really helped me when I was writing my PS was to never lose sight of the main goal of the whole essay...that is, show the admissions committee why you want to do medicine, what you have done to reinforce your decision, and why you are worthy of an acceptance. Showing, through anecdotes, is much more effective than just stating that you like helping people. Just remember, this is your absolute best shot to add a little personality to your application and to sell yourself to potential med schools. Good luck!