Inhale or exhale sharply, forming the word: YOU
Yes, I know it sounds stupid, but: YOU
Your facial structure will come alive, your cheekbones and mouth will look more prominent, and the look will be overall more engaging.
Okay so I got a few requests to elaborate/demonstrate this concept.
The rounder your face, the more you want to get into the word YOU. Try inhaling while physically saying the word aloud. Looks idiotic, but when you inhale sharply, your body contracts and your posture improves, your neck moves back, and you will look better overall. You want to think about bringing out the natural angles in your face. A flat expression does nothing for anyone. If you have a naturally thin or narrow face, you can relax a bit and inhale or exhale with less force.
I have a round/acorn-shaped face, so I tend to do as much as I can to bring out the angles in my features. I took a brief webcam video inhaling and exhaling on YOU and took the best ones out, as a photographer would do. Here they are (yes, lol, they’re not glamour shots, it’s late and I’m at my desk):
Here’s me doing it in an actual photoshoot for cosplay:
Face looks tighter, mouth has a nice shape, and the muscles in my face are engaged, plus my lips form a nice little bow. I’m not talking about OVER enunciating, I’m saying, whisper the word YOU on a breath out and then on a breath in, and watch your face get ten times more engaged in the shot.