I've written a quick guide to help conduct a successful paper prototyping session. You can read it here.
While no guide can ensure success, I've tried to address some of the common points of failure we've experienced. The most important thing to do is get the right people in the room, but the rest of the article covers tools and techniques to make sure the ideas are modeled effectively and communicated clearly.
I welcome your feedback on the article and hope you find it helpful.