Counting might help you to maintain a baseline breathing pattern, but I don't think it's necessary. You might try practicing and see what works best for you. While soothing thoughts might help keep your heart from pounding, remember that in a "control" question "test," the key to passing is to produce stronger reactions to the "control" questions than to the relevant questions. It's not necessary (and might even be counterproductive) to try to prevent reactions to the relevant questions. Instead, one should augment one's reactions to the "control" questions, as explained in Chapter 4 of The Lie Behind the Lie Detector.

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ROFL!  Sad part is, if you do that, you probably have a good chance to make headlines in newspapers that you are such an atrocious liar that the contact with the polygraph machine almost killed you.  Imagine your chances of employment with any law enforcement agency should something like that happen.  But nonetheless, you've got a good sense of humor