Sometimes it's due to your jaw shape and you can do nothing about it at all unless you have jaw realignment. All the exercises in the world won't put it right.
I thought I had a 'double chin'; however, during a routine physical my gp wasn't happy with the feel of it and sent me to an ent specialist. Turned out it was a cancerous growth. It was removed three years ago and so far everythings ok.
not sure if it works but you could try geting a foam ball, place it under your chin and squeeze it down towards your chest and hold for a few moments with presure then keep repeating