Forced emotions

[Mari]

As some of you may know, I've recently started working as a waitress/hostess/valet/kitchenhelp. At one of the places I worked was a poster on the kitchendoor saying "smile, you're going on-stage". When I'm working I have to not only smile, but be happy. Smiling is not enough, you have to show a smiling, cheerful, bright, full of energy kind of attitude. Off course I'm not always happy, most certainly not after 10 hours of work when an annoying guest starts telling me how bad they think the food was or whatever.
I think everyone has been in a situation where they had to/ were expected to show a certain emotion. I was wondering what kind of situations you guys have been in and how you handle these "expected emotions". I was also wondering how you got yourself in the right mood for showing an emotion you don't feel at the time.
How do you get to switch emotions?
I sometimes really have trouble being happy and keeping a smile on my face, especially since my natural expression isn't a happy bright one. Any tips?