The answer is unbelievably simple- FACIAL EXERCISES AND CHEWING.
If you take a look at the opening credits to Breakfast at Tiffany's, you will notice there is a scene where Audrey Hepburn is in front of a shop window and eats a Croissant. If you look very closely you will see that as she chews there is an immense layer of muscle around the sides of her jaw line, much too toned to just be there, she must excercise the muscle (s) !
Audrey Hepburn is not just the only Hollywood Star to have mastered the technique of chewing.
Take Brad Pitt for example, notice how in interviews and premieres he is always chewing?
If you look at videos and pictures of Brad Pitt in his early days (Dark side of the Sun) you will see that his jawline was much thicker then, e.g. more circular around the sides. The circular look is there due to pouches of muscle.
Yes its true that some people can (if they're lucky enough!) be born with a defined pair of cheekbones and a great jawline, but its more than POSSIBLE for the average person to match and even surpass them!
HOW TO EXERCISE FOR A GREAT JAW LINE (BOTH MEN AND WOMEN)
1. Take three pieces of good chewing gum (preferably sugar free) and begin to chew on you left side very slowly; feel the muscles working. Concentrate on chewing all around your left side (putting the gum right at the side of your mouth is good for building muscle). Do this for 5 minutes.
2. Repeat this on your right side.
3. Do this 4 days a week, gradually increasing length of exercise until you can do 12 minutes a day (Never do more than 15 minutes each side; the exercise should be quite intense).
The Jaw exercise above will add muscle around the sides of your jaw, defining your cheekbones naturally.
a) If you feel that your joints are aching, then STOP exercising immediately. Rest your Jaw and start again, remembering to perform the exercise very slowly. Make sure that you can feel your Muscles working NOT YOUR JOINTS.
b) Never perform the exercise more than 4 days a week. Your Jaw Muscles need time to repair them selves.
GOOD LUCK CHEWING!