Interesting solution that I just tried.Try this experiment:
Open your mouth as wide as you possibly can. Put your finger on your jawline there. Now close your mouth. Your finger should now be about an inch BELOW your jawline.
Do the same when shaving. Open your mouth very wide when shaving your cheeks, and then shave down to the jawline like that. When you close your mouth you will see that you won't have to shave the actual jawline itself.
Try the Joker's grin from the 40s-50s era Batman comics.