I was just reading a photographer today who said: get the subject bored. She suggested making them stand around, and give them a lot of directions -- move your feet, turn your shoulder, move your head to the right, eyes to me, etc. For about 3-4 minutes. This gets them over the "someone is taking a picture of me" and they relax, then you can shoot for a few minutes and they look more relaxed and not so anxious.