A headshot is one of the most important things for an actor to have. It will be your "face" for casting directors before you ever meet them. You want your photo to look as good as possible. While choosing a great photographer is important, you should not simply leave everything up to them. There is only so much they can do with lighting and camera angles. You also don't want to rely on Photoshopping or other post-session digital touch ups. You should look as good as possible so that the photo is "real". Casting agents will be turned off if they think your image is touched up and will pass you by. You don't want to risk it, so just make yourself as good as possible for your session. Here's some things to do.
Visit A Salon For A Nice Hairdo
This should go without saying, but you really need to have your hair done professionally by a great stylist. If you normally don't go in for expensive treatments such as Brazilian keratin, now is the one time you should. You want to look amazing for the photo since it will be the first time many casting agents see you. And this goes for men as well. Don't make the mistake and think that it's only women who need to hit a stylist. You want to skip the barber and visit a nice salon. Even if you're rocking the Jason Statham or Stanley Tucci look and don't see what a stylist can do with a bald/beard combo, you should still visit the salon and have them trim up your beard. Odds are a trained stylist will improve on your beard and make it super even and attractive.
Consider Bringing a Makeup Assistant Along With You
You can't get your makeup done the day before, and you don't know how the photographer's lighting will be set up. So, you might want to bring along a makeup assistant. They will know how to apply the correct makeup so that it looks amazing on camera. They can see the photos as they are being shot (go over the shots during the session with the photographer on his laptop), and they can adjust your makeup to make you look amazing.
Again, this one is not just for the ladies. Guys need to have makeup, though much less than ladies. Without makeup, skin will shine and might look too glossy. It's especially important if you have that above-mentioned look (the Statham bead/bald combo) and even more important if you don't have a beard ala The Rock or Bruce Willis. You don't want a bright, shiny dome.
A casting agent will likely pass by yellow, stained teeth. An actor with perfect pearly whites can have their teeth yellowed for a role requiring it, but making yellow teeth white is not something you should expect makeup and wardrobe to handle. As an actor, you are expected to have white teeth. And you want to smile in your headshots, so you need to visit a dentist and remove all those coffee, cola, and red wine stains. Teeth whitening is a procedure that will help you out in your auditions as well. For some reason if you do end up having headshots where you keep your teeth hidden, you will still have to meet face to face with casting agents and they will see your teeth and they will get turned off. So get it fixed and have one less thing to be nervous about while going on auditions.