Something that really helped me was reading some sample personal statements:
http://www.amazon.com/Essays-That-Will-Medical-School/dp/0764142275
Obviously, yours will be (and better be) very different from anyone else's, but through reading those, I found what I liked and what I didn't like in other people's essays. Some things I found that I liked:
-Essays with a theme that runs from beginning to end
-Unique essays--after reading a few, I realized how quickly they all start to sound similar. I found that I liked essays that discussed a person's non-biology major or minor, a person's unique research or volunteer experience, a unique hobby, etc.
-Essays with a hook that isn't stupid or desperate. I think this can be done very well if you launch immediately into a story (about a volunteer experience, research, a certain patient). I know opening with quotes is perhaps cliche and kind of frowned upon, but I did it anyway. I opened my essay with a short, simple, and bold quote by a famous philosopher, and I think it worked out well--several interviewers commented on it, and it led to some very interesting discussions during my interviews. Plus, this quote provided a unifying theme that tied together/summed up all of my motivations for a career in medicine. Warning though: don't do anything too flashy ("It was a dark and stormy night and there was a pool of blood on the floor of the ER..."). There is a fine line between interesting hook and extremely cliche, "interesting" hook.
Maybe one good place to start is with your activity descriptions--you'll have to fill them out anyway. Make a list of all of your activities, write a short paragraph about what you did in each one, what you learned, and what it meant to you, and so on. Figure out if you can see a theme emerging from a few of the most important activities. If you can, great! Try to expand on it. Alternatively, make a list of all of the reasons you want to be a physician. Say, helping people, intellectually stimulating, collegial profession, the impact of health on happiness, etc. Choose the single best, most original one and make it your theme. Expand on why its personally important to you and back up your claims with concrete examples from your experiences.
A word of caution--make sure your PS isn't just a list of your activities. Those essays are boring and not very persuasive.
