My two cents:
First: Go to the websites of the schools to which you want to apply...look at their criteria for selection and their mission statement; anything that clues you in about their focus and vision for the fundamentals of "their" type of physician.
Next: Go to Barnes and Noble (or any big bookstore) and look through a few of the books concerning this subject...there is a litany of them, but also look in the MCAT prep books, there is usually a section about the personal statement.
Finally: Using the particular qualities of the schools to which you've decided to apply and the information in the books (structure, content, example essays) formulate yours. I would say it is a good thing to do weeks, if not months ahead of time. You want it to be as polished as possible (I tried mine out on friends and family to make sure it was accurate and captured the essence of how I came to where I am today and the forces that drove that progression - the key here is essence, not facts, capture the emotion, put them in your shoes).
I don't know if that is wise advice. It is what I used and I have yet to be able to report just how fruitful it was, but I wish you well and be diligent, all will workout just as its planned...