Open your mouth into an O shape. "Wrap" your top and bottom lips over your upper and lower front teeth.
Tighten the muscles at the corners of your mouth. Imagine they are little knots that are very, very tight.
Keeping your lips tightly over your teeth, make a scooping action with your lower jaw, as if you are trying to scoop up icecream from a bowl. Make the scoop movement by consciously working the muscles at the corners of your mouth. You will look very silly 
Repeat 2x a day, with 40 repetitions each time.
After the 40 reps, hold the jaw still in the scooping position for 40 seconds.
Do all this daily for 3 weeks, then drop to 3x a week.
I did this, and my DD (20) thought I'd had "work" done on my face (she was at uni and hadn't seen me for a month).
I think it makes a difference and it may work for you too.
I have lent to book I got this exercise from to a friend, but if you gooogle facial exercises you'll find lots of offereings. I think my book was by an American with an Italian name.