Not exactly on my end either. I despise being told to smile for any reason, especially if it's to have my picture taken.
In fact, in plenty of social situations, I just find myself not even smiling to begin with. Don't know how that makes me come off to the other party, but it's not something I can help...
Unless they, like, compliment my shirt or something. That gets a tiny grin out of me, I think.

For pictures though, since it makes me look slightly less terrible, I just try to think about something that's made me happy or that makes me laugh so that I give a genuine smile (and so I'm not badgered about my smile as well).