I've done a few and never failed one. Was told to clench my butt cheeks really hard to get a pass. Another tip was to put a drawing pin into your shoe and push down onto it with your foot as hard s you were able to tolerate. And if you search online there are a few other techniques you can use to ace the tests. Most interesting for me was being told I'd failed a test I was later told I'd passed. It's sheer madness.
They often ask you explicitly if you know or have researched methods to defeat the polygraph. It's a lose/lose situation because most people have heard of such things or DID research out of curiosity facing taking a test, but admitting it makes you look bad. You generally don't get the chance to explain yourself, and if you do it just makes you look more guilty. The entire process is designed to give an excuse to arrive at the conclusions the entity administering the test wants to arrive at.
So, I would have totally failed the test repeatedly, because I would keep insisting on fully explaining the circumstances under which I had heard about the methods but not actively researched them.
And I would tell them to check my shoe and see that there is no thumbtack there. And check my cheek to prove that I'm not biting it in secret. And otherwise expose all the other methods that I know of, and prove that I'm not using any of them.
I'm sure that I would have frustrated the hell out of them, and they would have made damn sure that I always failed the tests, every time.
For federal agencies, the seat pad is now standard. There are no public studies of its effectiveness. Regarding how the U.S. government attempts to detect polygraph countermeasures, see:
A TV viewer thinks "How about butt clenching?", then one of the characters in the TV show articulates this thought and other gives a response. What actually the response is usually does not matter, because TV viewer already felt some connection with the show when that question was asked.
Something that's kind of rough and uncomfortable would make the most sense, especially if you had to sit in some exact place for several hours. Discomfort would add to the stress.
