MythBusters did a segment on this and they where not able to demonstrate a way to beat the test that was reliable. In fact, I don't think any of their "test subjects" where able to do so. Can some people do it? I think so. But I seriously doubt *you* could beat it unless you are a pathological liar who just doesn't care anything about truth, ethics or morals.
The MythBusters "Beat the Lie Detector" segment was particularly bad, and the producers of the show should be ashamed of it. You'll find a detailed critique here. In peer reviewed research on countermeasures, about half of programmed guilty subjects were able to fool the polygraph after a maximum of 30 minutes of instruction, and experienced polygraph examiners were unable to detect the countermeasures. See the studies by Charles R. Honts and others cited in the bibliography of The Lie Behind the Lie Detector.