Make sure you discuss your needs with the photographer and let them know if you have a “good side” or features you wish to accentuate or downplay.
Make sure your makeup is clean and natural. Wear whatever makeup you usually wear. If you are a male, consider using glare-reducing translucent powder.
Let your photographer know the purpose for the headshot. Commercial modeling headshots emphasize likability and smiles; theatrical headshots are more serious; and a professional headshot needs to look “professional.”
Stay away from busy backgrounds, and don't wear any clothing with loud patterns or designs. Dress in clothing that makes you feel comfortable while you're moving around in it.