I've always said this and finally I had a chance to demonstrate it: The moderator should be seen little and heard even less. It is up to the candidates to ask the follow-up questions and challenge one another.
When I raise my right hand to you (just before the candidates arrive), ... that means, 'That's enough. Cool it. Let's go.'
Preparation is based on one driving force for me and that is to be relaxed enough to be able to listen to what the candidates are saying and react appropriately.