Same thing goes for a hairstylist. Maybe you don't fix your hair everyday but your headshot needs to look clean and professional, not disheveled with stray hairs going every which way. After all, your headshot is the first impression you give to any potential client.
The photographer does similar work on your appearance, but uses light as the primary tool to shape your face. Light that falls on your face defines its shape and texture to a very large extent. So by controlling the quality and quantity of light and the angles at which light falls on your face, a photographer can sculpt the contours of your features and make them look their best.
Another thing to think about is wardrobe. It's best to bring a couple of outfit options, preferably ones without any distracting patterns, graphics and/or logos. Avoid pure white or pure black as much as possible. You should be the center of attention in your portrait and your clothing should not detract from that. There’s no photoshopping out a bad outfit!
Let’s say you found your photographer, my advice to you is to be just as prepared as the photographer by considering your makeup, hair and wardrobe. It really can’t go wrong from there and I can guarantee you that you will walk away with fantastic looking headshots of yourself that you will be happy with.