Telling someone to “just be natural” in front of a camera almost never works – it is the fastest way to get a stiff, self-conscious pose. What works instead is giving people something small and physical to do, so their attention goes to the action instead of the lens.
I ask people to walk toward me and stop, adjust a jacket, look away and then back on a count, or laugh at something specific rather than “smile.” Movement between poses, not just the poses themselves, is usually where the best frame happens – I keep shooting through the transitions, not just the held positions.
Nervous subjects relax faster when they see the back of the camera early and often, so I show a favorite frame within the first few minutes. Once someone has seen one photo they actually like, the rest of the session gets noticeably easier.