Lighting is perhaps the most important aspect of a good headshot. Too much light will blow the picture out, making the model look like a ghost. Not enough light -- or a poor lighting setup -- will leave the model in shadows. You want to use at least three lights when shooting a model. Position your key light to one side of the camera, so that half the models face is lit up. Position your second light -- or the fill light -- on the opposite side of the camera. This will reduce and/or remove shadows. The third light -- or the back light -- should be positioned behind the model. This will illuminate the hair and make the model stand out from the background.
Using a bounce card will help smooth out the models face and reduce shadows, especially if you are photographing her outside. A bounce card can be homemade or purchased. Most photography shops will have bounce cards for sale. To make your own, simply take a square piece of cardboard and cover one side with tinfoil -- or another reflective type of material. Use it to angle light -- either natural or artificial -- onto the models face.
The background of any headshot should not be ignored. If you are the photographer, ask the model if he has a particular background in mind. For example, if the model is from San Francisco, try having a landmark from the city behind him. Or, you can shoot inside a studio and have a solid colored background. The key -- however -- is to have the background accent the model, not overtake the picture. If someone is paying more attention to the locale of the picture, it's not a good headshot.
Regardless if you are trying to be an actor for TV or film, make sure to have two distinct headshots. The first headshot should showcase your dramatic side. In this shot, you should not be smiling or giving any indication of levity. The second headshot should be more comical, featuring a big smile or some kind of goofy expression. If you're living in Los Angeles, New York or any other area with a lot of casting calls, having two headshots will be beneficial. Always make sure to look as similar as possible to your headshot. For example, if you have black hair in your photo, do not dye it blond right before an audition.