I read this great post over on Robert I. Sutton's blog, describing the research behind the smile-get happy-smile some more processes. Most people think they smile in reaction to a happy feeling; but research shows that you can bring on a happy feeling by just smiling.
At this time of year, why not just smile then? Heck, at any time or place: smile your face off and see what happens. To understand the brain mechanisms, visit Sutton's blog, he's researched and written about Zajonc and others who have studied this and it is worth the read.