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What Hoffer said. Acknowledge the awkwardness of it. Let them know that even you, when in front of, and not behind, the camera feel awkward.
I tell people to roll their shoulders, shake their arms, and smile. Big breath. A little oxygen does a body good.
I also tell people that usually when I direct/pose someone to do something or I put them into a situation, it's okay if it feels a little awkward. I remind them that what I tell them to do isn't something they'd normally do. I say, "I'm going to tell you to stand tall, chin out and down, I want to to cross this arm like this, point your toes and... You'd never normally do this. But I promise, it'll look great." After I snap the first few pictures, I show them a sample on my screen. I've never had anyone say anything less than "Wow!" or "awwwwww". Then the trust begins.
As for the very first thing I do with my couples, I "stole" a move from Jose Villa. He has the couple stand shoulders square to the camera. Hold hands. Close their eyes. And on 3 he tells them to try to kiss. People always miss. If they miss, they laugh. SUPER good candid moment. Also a great opportunity to laugh WITH them and build that friendship/relationship. Then they know that you're with them, and not analyzing them and being critical. If they happen to land the kiss, I run up, high five them, and tell them that almost no one can do that, and they get all excited.