People just don't smile enough. We don't mean to make you a grinning idiot or a used car salesman. The secret is timing. You need to turn it on and off at the right time. Careful here - this sounds fake, but allow us explain. You have pleasant feelings and thoughts all the time. You wake up and see a great day outside. You pet the dog. You read Doonesbury or Dilbert. You find ten dollars in a pair of pants under your bed. You go the gym and see an empty treadmill next to that cute single redhead you've been interested in. (Now we're getting somewhere.)
In all these instances, especially if you are alone, you would just enjoy the moment and maintain your typical poker face. It's easier. What's the point of working out those facial muscles if no one can see it? Plenty. It's conditioning. Allow yourself to express warm and fuzzy feelings by smiling when it feels right. Always. People will actually begin to catch you at it. They will wonder what you're up to. After a while it will become second nature. You will begin to smile more. Of course, when the moment is over, you need to relax. People will think you're doing a Jack Nicholson impression if you don't.