there is an EXCELLENT book called essays that will get you into medical school. look at it in the library or browse thru it at your local b&n.
id suggest writing down on a piece of paper all the things that you enjoy doing, all the things that motivate you to become a doctor, various significant experiences you have had, any amazing achievements, hardships overcome, lessons learned, favorite movies, foods, music, etc. none of this has to be earth shatteringly amazing. this, in a few sheets of paper, is you.
talk over things with a friend and think about who you are and what motivates you to become a doctor. if there is a common thread thru your life, such as a passion for photography or mountain biking or mountain climbing, relate that to medicine and how the lessons you have learned or the things you have seen are similar to medicine. one AMAZING essay i read had to do with a climber and how she sees medicine as a series of climbs - no one becomes a climber or a doctor after one trip or one experience.
i think the most important thing is to be sincere. show how you were drawn to medicine, and dont say you want to help people or that you're a leader, show and explain how you are a leader or how you were born to be a doc. i also think that telling a story and explaining the significance of it or just reflecting on it's lessons can make a great essay.
but thats just air. i think you should get the book and look thru it. it has excellent essays at the end of the book, each for a different person - a research type person who volunteered in china, or an engineer with a research interest. most of these kids got into harvard(the author was an adcom member at hms i think) so the stories are insane.
that may be my longest post ever. i really hope it helps you out. the ps is damn hard. just make sure to give it to all your friends and their mothers and ask for critique. ask them not to sugarcoat it. also, check out the sdn'ers willing to read PS's thread in the preallo forum.