We do a great one in yoga class. Just make sure you do this at home, with the drapes shut, and no one around, because you will look silly.
Sitting comfortably, look up at the ceiling, open your mouth as wide as it can go, hold that for a few seconds, then clench your teeth, again holding for a few seconds, then return your face to normal, and put your head back to it's usual position. Repeat that three times.
My yoga teacher is 60+ and has the youngest looking neck I have ever seen!