When you step in front of the camera for a photograph, it’s almost second nature to kind of hunch up and put your chin down–turning a regular chin into a double chin, a double chin into a quatra-chin (you get the picture!).
When you stand in front of that camera, take a deep breath or two as this relaxes you, relax your posture (but maintain good posture while doing it!), lift your chin off of your chest, then imagine you are stretching your neck or pulling your head up using the hairs on the top of your head. This will give you a much more pleasing photograph and a more subtle and slender chin.