[vprao]

Keep smiling! Once you start practicing it, your sense of humor will automatically improve and the slightest humor will make you laugh.

OK, now if you find it difficult to smile too, then here's an exercise I absolutely love, being a serious person myself.

Just stand in front of the mirror and smile. Just watch and don't judge anything. Keep the expression for as long as you can and are comfortable with. Repeat it as many times as you can. You can also try raising your arms (like the laughing buddha). Raising arms can bring a natural smile without much effort. Try it and see the difference.