David Travis provides 15 useful hints for anyone conducting a usability test.
Running tests can be challenging for the facilitator. Knowing how to get the participant to share their thoughts, and then knowing how to listen to them is not something that necessarily comes easily.
Personally, I found recording my sessions to be enlightening. Not just to re-hear what the participant said. The way I phrased questions and gave the participant the space to think aloud was fascinating on hearing it back, and gave me food for thought in future sessions.
What every usability test moderator ought to know about good listening - article by David Travis