FOR MEN: A jacket is preferred (blue, black or grey), but not required to make a great headshot; a white undershirt can be worn underneath a dress shirt or polo, but definitely not a black one.
FOR WOMEN: Avoid overly glossy or shimmery makeup, as it will catch the light and be a distraction. Use powder to eliminate a shine, and if you normally don't wear makeup, your pictures will look a LOT better if you at least apply a light foundation. This helps even out the skin tone and makes it much easier to edit in post-processing.
FOR EVERYONE: Because you are representing your business, you want to convey the best impression; wear an outfit that speaks to your professionalism and the level of service that you provide to your clients. Keep it classic, simple and clean, PEOPLE! :)