For ladies it if you are facing straight at the camera look down slightly and to the right and then lift your eyes to look directly into the lens.
Some people's colourings lend themselves well to being photographed aswell and when someone who looks quite plain in the flesh their skin tone may actually flatten a flash better.
The majority of 'scary' photos are because of the limitatiions of small flashes on a camera. Models look nice in pictures mainly because they are lit well and the camera is exposing correctly.
If you snap someone in a dark nightclub with a little snappy camera the chances are most people will look scary, especially if they are well made up or, to be frank, a little sweaty. Once again this helps 'plainer' looking people - if a person's features are quite subtle then they are less likely to be caught in a 'bad light'. For someone who is quite distinctive looking the camera (and the person taking the picture) has to work that little bit harder to catch them in a 'good light'.
Does that make sense?