NPs get off on "pushing your buttons."  Try not to react.  They love that.  The other golden rule is never engage.  They love it when they are lying and spinning the truth in conversations.  If you don't engage in the conversation, the spin out of control.  Most importantly, don't feel sorry for them.  Its always a trap.  They count on your sympathy, and then they go in for the kill.  They can go from really nice to really mean and wacky at the flip of a switch.  DO NOT ENGAGE.  Think of a phrase that works for you when you don't know what to say - something like, "I wouldn't know about that", or Gee, I it's really nice outside, isn't it?"  They love to compliment just before it gets started.  One of my favorite stories is:

A little girl was walking through the woods.  She came across a snake in her path.  The snake said, "Please, please, its cold out here and I am freezing.  Put me in your coat and keep me warm, otherwise, I will die."  The girl replied, "But you are a snake, and you will surely bite me."  The snake said, "I won't bite you, I promise.  Just please put me in your coat and save me."  The little girl felt guilty and sorry for the snake.  So, she picked him up and put him just under her coat.  Suddenly she felt a sharp pain.  She pulled the snake back out from under her coat and said, "You bit me!  You promised me that you wouldn't!"  The snake replied, "But you knew what I was when you picked me up."