One thing that I find that helps, especially with headshots (that you don’t see the whole body), is this: I have two stuffed monkeys that hang out on my camera bag. When I can’t get the subject to relax, I’ll throw the monkeys at them and have them hold them (just out of frame). I’ve found that when people have something in their hand (especially something as ridiculous as some bright colored stuffed monkeys) they relax more, plus it takes their mind off how much they don’t like or want their picture taken. On very few occasions has someone not laughed or relaxed some from me throwing monkeys at them! Sometimes all it takes is for you to get them to laugh for them to loosen up. This may not work for all, but it works for me.