The data nerds at OKCupid cataloged over 7,000 photographs to find out what kinds of profile photos generated the most interest from its members. As expected, men and women should employ different strategies. For instance, it's good for young men with good abs to show off their stomachs, but its creepy for older men to flash their six-pack. For women, cleavage shots are always good, but they're even better for older women than younger women.
There are differences when it comes to smiling and looking at the camera, too:
Now, you're always told to look happy and make eye contact in social situations, but at least for your online dating photo, that's just not optimal advice. For women, a smile isn't strictly better: she actually gets the most messages by flirting directly into the camera, like the center and right-hand subjects above.
Notice that, however, that flirting away from the camera is the single worst attitude a woman can take. Certain social etiquettes apply even online: if you're going to be making eyes at someone, it should be with the person looking at your picture.
Men's photos are most effective when they look away from the camera and don't smile.