The FAQ has a small section on smiling in photographs, you could look at those links for some discussion.

A posing technique that I found pretty interesting when reading about it was the idea of the birdie. Nowadays when people are taking pictures of children they're free to take as many as they want (and immediately review them) until they get one that satisfies them. But in the days before roll film, when each heavy photographic plate had to be loaded into the back of the camera by hand, it was very frustrating if a young child didn't look at the camera. So photographers had little toys, often birds, that would mechanically flap their wings to get a child's attention. People still hold up toys for this purpose and some blogs say to paint your flash with bright colors if you're a family photographer, but since high quality cameras are ubiquitous it's not quite the same as the birdie.

The clamp for holding someone's head in place in the 1800s photo studio is widely known, but the general explanation is that it was for the long exposure. My posts in the link above talk about how long exposure wasn't entirely the case for the lack of smiling, because the technology was better than a lot of people realize. The clamp, then, served the purpose of keeping the head still when the focus was set. With view cameras, you stuck your head under the cloth to focus the image which was projected on a focusing screen, then you emerged and put a plate or film in the camera. Once you did that, if your subject moved, they would be out of focus and you'd have to take the plate out again. So you held their head in place with a cleverly-concealed clamp or rest. Again, this problem is largely solved by modern autofocus systems, though some fine art photographers use view cameras today.