The YC interview is so quick you are going to be out of there before you realize it has started. In other words, be on top of your game. Anticipate questions, especially the ones that you don't want to answer about the weakness of your product (they will come up). Don't be defensive. Have a demo ready to go when you walk in the room which gives a quick and good first impression. It's not going to be a lot of idle chit-chat, they are going to dive right in.