Moisturize your skin! Aside from coming to your headshot session rested and hydrated, coming moisturized is the single best tip I can give. Whether you use coconut oil, raw shea butter (my favorite), cocoa butter or extra virgin olive oil, This includes your lips. Some Carmex, Burt’s Bees or Chap Stick is wonderful to have in your pocket. Standard moisturizers that you find at drug stores are mostly useless.
Moisturize your hair! I have such dry hair that I personally use shea butter as well as a leave in conditioner. Moisturizing your skin and hair increases highlights on camera (but in a good way) and adds more dimension and life to the headshots.
Eye drops (to remove redness). It feels like every tip is the ‘most important’ but having clear white eyes feels like the most important. I could make corny, cliched comments about the ‘eyes being the window to the soul’, but your eyes are the focal point of headshots. Eyes can certainly be retouched to remove redness and veins, but the results are never as good as coming in with clear eyes.
Drink lots of water and avoid salts or breads. This may be one of the more common tips you’ll see, but retained water can present as puffy, less sculpted looking cheeks.