Raise your head up. Pretend that you are looking at something slightly above you.
Move your head forward. If possible, use strong (but not too strong) light. Light can hide blemishes.
Make sure the camera is focused 2 inches above your eyes. Tilt your head down and extend your neck. This stretches the excess skin and reduces the chance of a double chin appearing on film.
Do one of three things recommended by Jeff Smith, author of "Posing Techniques": 1) Hide your double chin by resting your chin on your hands, arms or shoulders. 2) Stretch your chin area by turning away from the main light. 3) Extend your chin and lower your entire face.
Take pictures from a side angle and above your eyes or head.