Push out your forehead
You'll feel like a complete idiot doing this, but tipping your forehead towards the camera with your chin slightly down can do wonders for any headshot. It's a foolproof way to avoid capturing a double-chin and it draws the eye to your face from a more flattering angle.
Always be squinching
Halfway between a squint and a wide-eyed stare, the "squinch," where you bring your lower lid up just slightly, will instantly make anyone more photogenic. It takes some practice to truly master, but professional portrait photographer Peter Hurley has a spectacular video outlining exactly how to do it well.