This might sounds kind of unethycal, but giving them a drink to relax seems to have done the job for me quite some times,
i sometimes even hand the camera over to them to let them take pictures of me (only when i trust them) and that works quite often, i dont mind at all having my picture taken so its not really a problem for me. Also try to "teach" them a bit about portrait photography and then they do start to take some intrest in it they will sort of understand what your going for....
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