The key to outsmarting a polygraph, according to the outstanding citizens who specialize in such matters, is understanding what the machine measures: your heart rate, blood pressure, and breathing. Keep those factors constant from the control questions (the easy ones which establish your normal baseline reaction) to the potentially incriminating queries, and you're all clear.
Think the trick lies in learning to decrease your response to the stressful questions? Nope: Polygraph professionals say you want to increase your response to the easy ones. When answering control questions, you want your body to freak out a bit -- bite your tongue, for example, or, erm, flex certain interior posterior muscles -- to skew the baseline measurements and throw everything out of whack.