The polygraph is a big hoax, anyone can easily manipulate the results of a polygraph by using easy to learn countermeasures.
What a polygraph does is measures sweat on the finger tips, heart rate by blood pressure, and breathing with tubes around the abdomen. Then it records each measurement on paper.
How it works is the "polygrapher" asks control questions, relevent questions, and irrelevant questions. To determine whether you're lying, he will compare the control questions reactions and the relevants reactions. If the control questions are more reactive then you are telling the truth, if the relevant question is more reactive, then you are deemed lying. This is the ONLY way a polygraph can be done.
So pretty much, if you want to defeat the polygraph; all you have to do is react to the control questions more than you react to the relevant questions. This is explained in a free downloadable book on the site
http://www.antipolygraph.org/ and it shows you exactly how to do it. I suggest reading it just for information on this voodoo machine.
Pretty funny how so many organizations put faith into this machine when it is just a big load of BS. Anyone can can beat it, yet these departments still use it. It also gives false positives sometimes too which can say someone was lying when they weren't, and this happens around 50% of the time. This can ruin peoples careers with organizations such as the FBI, even though they were telling the truth. Pretty scary.