Here's a tip I give people. It really helps a lot if you tell people how you are going to conduct the actual shooting part of a photo shoot. A lot of people are nervous in front of a camera. First off, when fiddling with the camera, TELL people, "I am just testing the exposure" or "I am just checking my lighting set-up". Let them know there is no need to pose or to smile or to be "on". Years ago, we were often told to shoot the first five minutes with no film, to help people warm up. That saved $15 right there, but in the digital era, it's dumb.
It helps to EXPLAIN to people, directly, with comments like: We're going to shoot for 40 minutes. What we will do is set up a pose, and then I will take 3 or 4 or 5 shots, and then I will tell you how to move into the next variation of that pose, or we'll set up a brand new pose. Not every shot has to be perfect. We are shooting for a 1 out of five ratio here. We want a variety of expressions. Not all smiles, not all pouts, not all formal...we want a mixture of different expressions. Not each shot has to be a big, cheesy smile.
If you TELL people what I just told you, you will be able to get people to loosen up fast. If they do a pose or expression that is good, say "Great! Looks really good!" so they get some positive feedback. What this does is it reduces some of the fundamental fears people have, and it clearly sets the expectations. Many people want to make each shot "perfect", which is an utter disaster waiting to happen. A subject worrying about making each shot "perfect"almost always leads to boring or even bad expression and posing. You must tell them that not every shot is expected to be "perfect".
If a person is not "in the groove" by 10 minutes, take a break and show them a quick LCD review, and give them some positive feedback on some good shots, so they can literally see what they are putting forward, and what you have been trying to achieve.