No matter how much you want to bolt for the door after an interview, look your interviewer in the eye, shake hands, and say thank you when you're heading out. Most people immediately notice when manners are amiss and you want to leave a lasting impression of politeness. Take your graciousness one step further and send an individual thank-you note to each person who interviewed you.
An emailed note is sometimes appropriate, but a hand-written note is a bit more memorable. Place the blank cards in your leather-bound portfolio and sit down at the coffee shop down the street to write them just after the interview. When they receive your cards the next day it will be obvious that you thought ahead and are taking the opportunity seriously.