Posing the head
Even small adjustments can have a big impact on the look of the face. All subjects are different, but in general a slightly side-on stance works well.
As you can see in these profile shots, asking the subject to roll back their shoulders and push their head forwards slightly will tighten the skin and reduce double chins. Camera height is important too: having the camera level with the eyes creates a strong connection. As with any portrait, it’s vital to put the subject at ease, so chat to them as you shoot, perhaps put some music on to help them relax and, vitally, offer lots of encouragement.