@darix recently pointed these out to me, and they are definitely worth a post here.
For those not familiar with Peter Hurley, he’s a well-known headshot photographer out of NYC that has taken the time recently to share some wonderful tips about shooting headshots.
The first of them emphasizes just how important the jawline is to a headshot. There really are some wow moments in this video as you see what he gets out of his models (the difference is striking):
The second video is about what he calls squinching, or the process of squinting with only the lower lids of your eyes (mostly):
I love these videos, and have made use of his tips many times. I found them to be particularly helpful in my own headshots that I use.
Here is one of my previous headshots before I really tried anything from these videos:
And here is the result after paying attention to the tips and applying them to a new headshot:
I may not be pretty, but I think that’s a better result than the previous image, imo… 
