If it is a friend, crack open a bottle of wine. Have some laughs and catch her just after the laughter has died down but there is still that bit of genuine smile left in her eyes.
For paying clients that may not be the perfect solution though. There it comes down more to your personality and how you can manage to get them to relax. Expect to discard most of the first frames. Classic trick was after all to shoot blanks for the first “roll” but that is hardly worth doing now that we don’t have to pay for film.