All it is is a stress test. Just don't sweat it and you will ring true. People get nervous as they lie, and certain things happen to your body when you get nervous.
Several reasons that cause them to be inaccurate.
To some people lie is very easy and they have no fear of getting caught, or that they will.
If you don't believe they work, sometimes that helps since you don't get nervous as you lie. Most people get nervous even when they don't think they work just because in the back of their mind is the WHAT IF
Some people are so nervous the damn thing can't differentiate between lies and non lies. They are just nervous as hell even while telling the truth.
One suggestion is if you know of something that makes you nervous (such as speaking in front a large audience) work on fixing that in turn that will help you control you nervousness for polygraphs.
Look up about the sings of being nervous, I think there pulse rate, sweat and voice, are the biggys, If you can control those things. hmm mabe DNP and ECAY will fuck it up, You'll be sweating like a bitch with a wicked high pulse. Though then they might think you hiding something like mabe that your on DNP