Before your testers arrive, set aside some time for everyone on your team involved in the playtest to do a run through. Testing your playtest will ensure things run as smoothly as possible, by ironing out the kinks ahead of time and getting the whole team on the same page.
The concept of playtesting is still new to some game developers; for the average human being, it is completely foreign. Your playtesters may be unsure of what to expect, and that uncertainty can have a negative effect on your test results. Make your playtesters comfortable so they can focus on playing your game and you'll get much better feedback.
Be sure to tell your moderators to segregate themselves from the game during the test. We assure players, "We're testing the game, not you," and that we are hoping to find areas where they struggle or get confused, because we want to improve the game. We don't want our players to be afraid of giving honest (negative) feedback, so we always stress that we are observers, not designers, and are here to listen to any and all criticisms.
Our observers sit back and let the playtester play the game with very little interference. However, we find it helpful to encourage the playtester to think out loud during their experience, to help us understand their point of view.
Recording the test for the rest of the game's team to watch later is also helpful. Because most people don't play video games in front of a note taker and a video camera, we find it's important to put a player's mind at ease about these elements and explain to them why they are helpful. Players usually enter our office not knowing what to expect, but leave understanding what a playtest is and why we conduct them.