I'm not an expert but I just talked about why I wanted to become a doctor and why I wanted to be a D.O.
I remember a thread awhile back on personal statements...you might want to look for it to get more answers if you don't get a lot of other responses.
my biggest advice: this is the one place where you can truly stand out from everyone else in the committee's pile. It's always a dispute about how much weight they put on those, but I think it can be a very important factor. It's the one interesting part of an application and can definitely bring up topics of conversation in an interview. DO schools really like conversational interviews from what I can tell.
So, the bottom line is, make yourself seem different--tell about your crazy hobbies, amazing volunteer work, or interesting childhood. Be unique.
ditto to anna's statements....be creative! one guy i know talked about how one of his greatest challenges in life was his voice! when you talk to the guy you can hear why it was totally different and creative and admissions loved it!