I'm no expert, but I can tell you how I mapped out my statement of purpose. First, I quickly touched on what events/passions led to the pursuit of an MPA. I then focused the majority of the writing on to what specific impact(s) I want to make in urban policy, both short and long term, and how SIPA would help achieve those goals. I tied the core curriculum and concentrations to my interests and I think it can be good to research the professors so you can identify one who works in the field you're most interested in. If you're including a professor then just make sure putting their name in has a purpose and is not just 'name dropping'. Finally, I put an emphasis on what I can bring to the classroom. Hope that helps and best of luck putting together your application!