Plan your route to the shoot, check the traffic, parking, etc. You don't want to be late. Often, photographers have another shoot scheduled after you and you don't want to be rushed and lose those precious minutes. Time is money, so be considerate.
Try on your outfits at home and make sure they are not wrinkled. Take various options to the shoot. Choose colors that complement you and garments that flatter you. Solid colors are preferred. Avoid: white tops (OK with a jacket on top), logos, busy patterns or too many accessories. Your eyes and face should be the stars, not the outfit. If what you're selling is your outrageous personal fashion style, then maybe you would be an exception.
KNOW YOUR BEST LOOKS
Most people know their angles and the expressions that look good or bad in pictures. If you don't, take a good look in the mirror and figure it out. Also, look at pictures of yourself and see which ones you like and why.
Speaking of face, ladies, try starting out with a natural make-up look and then you can build up from there. Practice this transformation at home so you can change your look quickly. Remember that heavy eye makeup ages you and sometimes, less is more. If you need help with make-up, let your photographer know ahead of time as someone will have to be hired for you. Budget for the service of the makeup artist.
Men, you usually look better without makeup, unless you're a rock star and eye liner is part of your "look". Perhaps you'd like some photos with facial hair/stubble and then shave (if you can do it quickly). Bring what you need for your transformations.
Ladies, consider putting your hair up for a different look. Men, also try different hair styles, such as adding hair jell toward the end of the shoot.
GET SOME REST - There's no fixing it in post.
Beauty sleep, really works!!! A lot of makeup cannot hide your lack of sleep when you've been out drinking 'till 3AM. You don't want to do heavy editing of your headshot because it could look phony and be apparent and the photo should look like you. Actors, casting directors can usually tell when there's too much retouching and they won't be happy about you misrepresenting yourself so you might not be invited to the audition. Even business photos shouldn't look like you, 15 years ago, right?
COMMUNICATION IS KEY
Tell your photographer what you want to do with your photos and what look you're going after. Listen and follow suggestions and direction. Remember that your picture needs to communicate without words. Your eyes have to tell a story, the face and body have to support that story or feeling. If you relax and have fun interacting with the person on the other side of the camera, your pictures will reflect that and you will get outstanding results.