My sister used to do face exercises all the time. She had a couple she did, and explained them to me.
One of them is merely a series of extreme facial extortions flowing nonstop into eachother:
Make your mouth into a very hard O shape.
Open your jaw (not your mouth, your jaw) as far as it will go.
Then open your mouth as big as you can.
Stretch it as wide as you can get it without closing your jaw.
Continue widening your mouth into an extreme grin, allowing it to pull your jaw closed.
Retturn your mouth to the hard O shape.
Just keep doing these repeticiously.
The other is for the double chin, and is simply pushing the point of your tongue backwards to your throat as far as you can. You'll feel the tightening in the under-chin and know when you're doing it right.
Those are the only two I know of, and I'm not so sure that the first one won't contribute to premature wrinkling. Maybe someone will come up with something better.