Photographer Peter Hurley shares a tip for looking more photogenic in portraits: squinching.

Hurley is big on this: squinching isn’t the same as squinting! The difference is minor but important. When you squint, your top and bottom eyelids close up and your eyes end up all but disappearing — you look neither confident nor self-assured.
With ‘squinching,’ you’re lifting and tightening the lower eyelid, while only letting the top one come down a hair. It might seem like a slight difference (and it is) but it’s a big deal when you get in front of the camera.

The move is said to make you seem self-assured and confident rather than wide-eyed and uncertain, and there are some before/after pics at the link to illustrate the difference.

I've tried it with great results, take a look here.