The truest smile happens when the mouth remains immobile. It's in the way the eyes fill up with positive energy, bursting to come out. And the muscles of the whole face take on a subtle "smile configuration."
There's really no way to fake that kind of smile. In any given moment, you either have it or you don't.
When the next genuine smile opportunity comes up, try smiling only with your eyes and keep that annoying mouth out of the picture.
Then, observe the impact.