Usability testing is nothing but testing done to evaluate the functioning of a product before it is released in the market. Usability testing can go awry if the facilitator is not capable enough to handle it properly. So before executing any kind of usability testing the facilitator (the person who is doing the testing) should bear in mind few points.

Do not deviate from your goal. The facilitator should be well aware of his responsibilities and that is to gather as much information as possible about the product or application. So it is upon his hands as to how he should maneuver the whole course of the testing. He can make it a success or a failure. It all depends upon how well he handles the testing procedure.
Try to know your participants. Before giving suggestions as to how to go about it, try to understand your participants. Some may grasp things very quickly while others take time. So deal with the participants accordingly.
Make them familiar with the environment. The participants may be or may not be familiar with the application, service or the product. It may not be new to some while for others it may be their first time.
Make the participants feel at ease. Provide an atmosphere where they can carry out the tasks in a relaxed mood. Do whatever you can to make them feel at ease. If they are cool and composed then only the testing would become productive.
Do not test the participants. Very often the participants do get a feeling that they are being tested instead of the product or software so they should be given the assurance that the product or service and not they are the purpose of testing.
Be a good listener. If you are a good listener then the participants would automatically feel comfortable. They will come up with suggestions of improvements.
Never intimidate them. Always give scenarios and never instruct them as nobody likes anyone bossing them around.
Do not rub them the wrong way. While carrying out the test if the participants are stuck somewhere or are not able to complete the task do not make faces or noises in a way to provide hints or your displeasure as it may hurt their ego.