You want your headshot to be flattering, but it must also look like you. It should not be overly-lit, hidden in shadows, or most commonly, over-retouched. You want the casting director to recognize you when you walk into the audition, and the headshot should make them excited to meet you in person.
Wear something simple that doesn't steal the show from your face. Clean, simple makeup that is similar to the makeup that you'll wear to the audition.
Professional printing is also important for a great headshot. It doesn't make sense to make the investment in a fasntastic photograph and then print it out at home before an audition. Presentation is very important in this business.