“A polygraph is nothing more than a psychological billy club used to coerce and intimidate people,” says Doug Williams, a former Oklahoma City police detective and polygraph examiner who for 36 years has trained people to pass the lie-detector test. The first step is not to be intimidated. Most tests include two types of questions: relevant ones about a specific incident (“Did you leak classified information to The New York Times?”) and broader so-called control questions (“Have you ever lied to anyone who trusted you?”). The test assumes that an innocent person telling the truth will have a stronger reaction to the control questions than to the relevant ones. Before your test, practice deciphering between the two question types. “Go to the beach” when you hear a relevant question, Williams says. Calm yourself before answering by imagining gentle waves and warm sand.

When you get a control question, which is more general, envision the scariest thing you can in order to trigger physiological distress; the polygraph’s tubes around your chest measure breathing, the arm cuff monitors heart rate and electrodes attached to you fingertips detect perspiration. What is your greatest fear? Falling? Drowning? Being buried alive? “Picture that,” Williams says. He used to advise trainees to clench their anus but has since concluded that terrifying mental imagery works better.

Williams, who is 69, may be among the more vitriolic critics of polygraphs, which he refers to as “insidious Orwellian instruments of torture,” but their reliability has long been questioned elsewhere, too. Federal legislation prohibits most private employers from using polygraphs. The U.S. Supreme Court has ruled that lower courts can ban them as evidence, and the scientific community has repeatedly raised concerns about their ability to accurately detect lies. Still, the federal government and state and local law-enforcement agencies continue to administer them. Last November, the Justice Department charged Williams with witness tampering after he gave his polygraph tutorial to undercover agents posing as federal-job applicants who had engaged in illegal activities. Even with a looming court date, Williams is coaching clients and crusading against “this dangerous myth of lie detection.” The government, he says, is really after him for exposing the test’s fallibility: “I’ve made them look like fools and con men.”