Before the Appointment
Rest - Get lots of rest. Do not go out the night before. Preferably. Don't drink for a day or two before the shoot. The bags under your eyes will show strain and there's nothing that can be done. It'll save you a greater retouching fee.
Clothing - Wear simple clothing. Black always works, one color v-neck t-shirts are great. This isn't the Rachel Zoe Project and you're not shooting the cover of Vogue. You want to communicate that you're an actor who can handle any role.
Make-up - If you can't afford a make-up artist. Go to Sephora right before your headshot appointment. Tell them what it's for and they'll help you out, all for the cost of some lipstick you're going to buy anyway. (Guys, you can skip this.)
Hair - You need a hair-cut anyway, book an appointment with your stylist, before you pop over to Sephora. That way you'll look stunning.
Teeth - Get a tooth whiting kit, they can be cheap but in the long run, you'll stand out with your pearly whites in full view. It'll be easier to retouch and retain more detail, the whiter your teeth are.
Relax - It's easy and quick. There's no need to stress, you're prepared. I'm prepared and we'll have a great shoot and take a good picture.
Think of a headshot as preparing for a role and having all your lines and props in place before you go on stage. All the prep is well worth it when we create an amazing headshot that get's you work.