I took a poly for a big secret government agency.  After spending so much time on this site, I was scared that I would be given a "false positive".  So I decided to employ the breathing technique countermeasure, by holding by breath ever so slightly during the certain questions.  Of course, I was interrogated and at the end of the session the polygrapher told me that he knew that I was "holding my breath just like they tell me to do on the internet" - were his exact words.  I was caught, busted, and my heart sank.  Of course, to save face, I denied using countermeasures just so that my denial would be on record.  But in my heart and mind I knew that I was caught.  Those pneumatic tubes they put on your chest are very sensitive and I'm sure the polygrapher could see every breath-hold that I did.  My dream job floated away after that polygraph.  I would advise everyone, if you must use a CM, use mental CMs.  Nothing can read your mind.  But your breathing, tongue bitting, ass squeezing, and other physical activity can be read.  With the sensors they put on you, including the seat sensor that can detect butt squeezing and farts, the mirrors, the high resolution cameras in the room, and the fact that your polygrapher is sitting right next to you to watch your movements, don't even try to change your physical activity if you can.  I tried to use the CMs that I used on Antipolygraph.org and I was busted.  I'm so ashamed and have to carry this for life. 
:'(