Someone on another thread suggested a book called "Writing About me," by Barbara Huntington and Linda Masse.
It could prove useful. I'm planning to pick up a copy because even though I'm a couple of years from actually writing my PS, I have no idea where to even begin.
Convincing an AdCom in 5000 characters (including spaces) that you're the right candidate for their school is a daunting task!
Good luck! 🙂