Many people freeze their face or try to hold their mouth just so in order to "take a better picture". These are wrong. The key is to control the lighting. The other day at work, a studio photographer came to take shots of new employees for insertion into the business page of the newspaper. A very ordinary-looking woman wound up looking very nice with studio lighting, no special make-up. Studio lighting fills in depressions, sunken eyes, softens things overall.
The sad news is, in candids, you won't ever be able to control lighting. Those on-camera floodlight flashes on the average point and shoot just ravage most faces.