A couple of different clients have recently asked me to improve their smiles in their portraits. Photoshop provides some clever tools for making subtle adjustments to the shape of objects, but what to adjust in this case? A little research helped me to quantify the intuition that most of us have about "genuine" smiles. Turns out that the right Photoshop tweak is to simulate contraction of the orbicularis oculi muscles, yielding a "Duchenne smile". This article provides a brief overview of the science.
(Of course, first and foremost, portrait photographers have to be good at eliciting natural smiles during photography. My goal is to avoid post-production tricks, but to be skilled at using the tools when the need arises. In this case, for example, one of the subjects had a medical condition which affected his ability to smile.)