Choose a photographer based on his portfolio and the kinds of headshots in which he specializes. If you are interested in being cast for feature films, avoid photographers who work mainly with comic actors.
Although a bit pricey at a range of $50 to $300, a makeup-artist is a wise investment. The camera picks up every little flaw, so having the right makeup artist will give your headshots an edge they otherwise lack. Visit a makeup counter with makeup artists whose work you like and ask for a "trial." You may have to buy a minimum number of products, but this is the best way to see if you like her work.
Stay away from bold patterns and logos that will compete for the viewer's attention. Keep jewelry to a bare minimum and try to wear clothing similar to what you would wear to an audition. This avoids a situation in which a casting director wonders why you don't look the same as the person in the picture. Take with you at least three changes of clothing for your shoot.