I have a good bit of experience in user testing with sites like UserTesting, Validately, and TryMyUI. I actively use the TryMyUI guidelines in all of my user tests. They are as follows:
To be a great usability tester you must:
Follow instructions closely and stay on task. You will be given a set of tasks to perform on the test website, and you must follow them carefully.
Clearly say exactly what you are thinking as you are thinking it. We are interested in your impressions, expectations, and the motivations for your actions. Don't edit your thoughts as you navigate the website; simply say exactly what you are thinking at each step.
Write a clear, detailed written summary answering some questions about your experience after you complete the video portion of the test.
IF YOU WANT A RECORDING OR A NONMODERATED TEST, IT MUST BE SELECTED IN THE EXTRAS.
My time for moderated tests is 4:30-9:00 PM EST during most weekdays, Noon to 10 on Saturday, and 2:00-9:00 PM on Sunday.