Use natural light whenever possible, ideally from one source so as to have the greatest opportunity to sculpt the face with shadows

Use these shadows to define the shape of the face and its features

Give the skin the opportunity to absorb the natural light fully rather than reflecting it as a glare

Set the sitter against a plain background as far as possible to give the head its focus without distraction

Relax the sitter with chat that reveals aspects/details about yourself to encourage the sitter to do the same in his/her expressions

Make your instructions instinctive and simple

Watch the sitter's eyes to judge the exact moment to take the portrait