For many people, sitting in front of a professional photographer having your headshots taken is much like going to the dentist - an evil necessity. The camera can quite literally suck the life out of people sitting in front of it - the chin gets tucked into the neck, the shoulders droop forwards and the eyes become expressionless, lifeless holes in the head.
One way to reduce the effect of double chins and neck wrinkles is to keep your chin up. Lean slightly forward at the waist and bring your forehead closer to the camera. From the side it will look like an E.T. impression but it will work wonders for your neck and jaw line.
Posing in such a way will feel a little unnatural at first and the trick is to get comfortable with it. This technique also works when being photographed from the side - you will then need to bring your ear closer to the camera and not your forehead. Light digital retouching will complete the process.