I was in your shoes last year. Read samples like everyone said. You can search google and find alot of good examples. After a while they all start to sound the same.
I used the following format, first paragraph--I introduced myself with something interesting, I talked about snowboarding. Second paragraph--How i decided on anesthesia/Why i like anesthesia. Third paragraph--my plans for the future and how anesthesia fits into that (i plugged my research interests). Fourth, i have a good personality/nice guy/hard working/interests outside medicine. Fifth--conclusion that ties it all together.
It must be less than 1 page, have your dean of student affairs critique it for you. I've been told that in general, boring is a safer bet, but I'm not sure I agree. A good number of my interviews started with questions about my person statement; about snowboarding or my research interests.