I have had people in the past ask me to remove their double chin or try to take photos in angles where you can’t see it.
If you go on a latter and shoot down at the subject helps to stretch their necks out or can have them push their faces forward.
Here is a tutorial I found online that might help others for the editing process in Photoshop.
How to remove a double chin
http://www.5min.com/Video/How-to-remove-a-double-chin-in-Photoshop-159163824