My mom is the same way regarding the "camera smile." I've only recently been able to overcome this, and I do so by telling her that if she can't muster a real smile I'm just going to keep clicking the shutter until she gets sick of me. I usually end up with lots of fake looking photos and then several at the end where all of my gentle prodding gets her to laugh and relax a bit. So despite a lot of advice to the contrary, I find that pointing out people's crappy fake smile sometimes is just the thing to get them to smile for real.
