Photographer Peter Hurley and many others tell portrait photographers to instruct their subjects to stick their chin out. When someone sticks their chin out, they elongate the area under their chin and therefore make their jaw line look better when it comes to taking a portrait.
So in order to take it one step further without making your subject visually uncomfortable, you can also tell them to bring their chin down just a tad. But then what do you after that?. thephoblographer.com