Originally Posted by Bart

A person who convinces himself that he is telling the truth can pass the polygraph. The polygraph is a pretty good litmus test for lying; however, there are a lot of factors that come into play such as whether or not the person is jacked up on caffeine, whether or not the person had a good night's sleep, whether or not the person is currently under stress (or duress), etc. The polygrapher needs to know these factors in order to establish an accurate baseline.

Otherwise, without a deliberate, systematic approach, it is, as you pointed out, voodoo.