A good place to start is to look at things that you have cut from your personal statement. I had to cut stuff out of my personal statement to bring it down to the required length, but ended up using some of the cut material for secondaries.
If you get really stuck do what my AP English teacher had us do in high school when we were applying to college. Just start writing! Don't stop to edit or think. Just right about why you want to be a doctor, childhood upbringing, what you want out of a medical school, random health care experiences (for those applying next year may I suggest keeping a journal where you make note of volunteer, extracurricular experiences that affected you). Then go research the schools that you are applying to!