The key in facilitating a usability test is to make the participant comfortable. Reassure them that they are not being judged/evaluated, the designs are. Have a quick chat at the beginning to put the person at ease and to get to know their personality more. Are they talkative? Are they curt? Getting to know the person helps you adjust your style to get the answers you need (for example, you may have to ask more follow-up questions to the person who doesn’t talk a lot).
Additionally, the questions should be A) about the design that is being reviewed and B) what the participant thinks about the design. Try not to ask any questions that are not related to those two things.
Be flexible – have standard questions you want answered but don’t be afraid to ask followup questions.
Finally, make sure you are not explaining the design. It’s not a walkthrough, it’s a usability test. The ratio of you talking versus the participant talking should be 5 or 6 to 1. It’s not about you, it’s about them and their reaction and use of the design. Listen, and follow up – you have two ears, and one mouth. Use them.