Be honest! We can't stress this enough. Don't try and say what you think the client wants to hear. Give honest feedback. if you don't like something, tell them.
Be polite! I always try to put myself in the mind set that I am talking to a good friend about this stuff. I also try and smile while I am doing it. Obviously they can't see you, but I think it comes across in your voice. The clients listen to a lot of strangers talking and a monotone or bored sounding voice can turn them off, even if the person is offering good feedback. They are also real people, so put yourself in their place and give constructive feedback without being rude.
Pay attention! Read and re-read the instructions on each task. Most tests have a specific scenario or mindset you need to be in, so try your hardest to test from that point of view. Make sure you understand perfectly what is being asked. If you don't understand what they are asking, say so and then say "This is what I *think* you mean" and give feedback on what you think they are asking. Take your time. Never try and speed through the test. The test should be the ONLY thing you are doing too. I once talked to somebody who was upset they received a bad rating on a test, but it was warranted because they were EATING while taking the test, lol.
Environment! Create a space to test in and make it comfortable. Being comfortable also comes across in your voice, more than you would think. Find someplace where you won't be bothered or interrupted in your test. Clients will often give you a low rating if there is a lot of distractions or extra noises in your test audio.
Equipment! The quality of your audio( and sometimes video!) can make or break you. You don't have to spend a lot of money, but get a dedicated headset or microphone. Most laptop microphones are just not up to the task. We have seen a lot of videos that might have given good feedback, but it was very hard to understand what the tester was saying, so we had to reject the test. Which is a huge waste of time for both you and us.
Tell us what you think! Don't just tell us what you see or just read the text on the website. Tell us how you feel about the content. Is it easy to read? Are the colors off-putting? Is it hard to find information? We can see the same things you do on the website being tested, but we don't know how you feel about what you are seeing. Tell us if you find something confusing or if they did a great job on presenting info. Remember we are helping people create better, more useful websites and applications. That is a win-win for everybody!