I agree with George Maschke on this one. Everyone I know that has taken a polygraph, myself included, gets accused of using countermeasures. It is part of the polygraph procedure, to accuse people of stuff and see what they will confess to. George is right. If you accuse enough people of using countermeasures, some people will admit to it. Others won't. Polygraphers are merely fishing in this regard.
I will tell you that the BEST countermeasures to use are mental ones. I mean really, common sense will tell you that if a person is biting their tongue, holding their breath, or doing something else physical but doing it incorrectly that it can be spotted. I can look at a person and see if they are biting their tongue sometimes. And those tubes they put around your chest monitor breathing, so if you don't employ countermeasures correctly, you can screw up. The reason polygraphers haven't come out with some sort of public report or video showing how they detect these things are because once they give away their detection method and show what the magical charts look like from someone using these countermeasures, then everyone will switch to a different countermeasure to try to beat the polygraphers.
Mental countermeasures work best, because nobody can read your mind. Which is why the polygraph doesn't work. Frankly, if you think about how much you want to kill someone the entire polygraph session, or think about sex, or all the dirty deeds you've done and not been caught for, you can appear to be having a "reaction" the entire time.
Anyway, I encourage everyone to use mental countermeasures if you choose to. They are the safest to use and less likely you will screw up. Read TLBTLD, and make sure you properly employ mental countermeasures. Trust me, they work. They worked for me.