Our thread on writing new playbooks over at Story Games is pretty good. You should check it out. Later on I will probably post some more things in it that I don’t see being talked about already.
http://www.story-games.com/forums/discussion/18311/apocalypse-world-writing-playbooks#Item_0
http://www.story-games.com/forums/discussion/18311/apocalypse-world-writing-playbooks#Item_0