It can be quite daunting sitting in front of a photographer in a professional studio. Typically, the shoulders droop forward, the chin drops to to the chest, the mouth forms a rictus-like grimace, and the eyes become expressionless black holes devoid of any life.
Taking a deep breathe, relaxing and pulling your shoulders back will immediately improve your posture. Double chins, and wrinkly necks can be improved by leaning slightly forward at the waist, bringing your forehead closer to the camera and lifting your chin up. 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.