Practice Moderating Sessions

Start with the right attitude. Do not be a perfectionist. You are going to make mistakes, bias participants, reveal information you should not, and invent new ways to invalidate a test session's results. This is just par for the course.
Usability testing has a twofold saving grace — testing multiple participants and iterative design Testing multiple participants means that if you invalidate  something in one session, there is always anothe opportunity to do it right.
Iterative design also makes up for any mistakes you might make, because you have several chances throughout the product development lifecycle to catch problems with the product. The important thing is not to get discouraged. Continue to practice, continue to learn, continue to improve. Even the most
experienced test moderators make mistakes.