Laughter is an internal aerobics, which will lower blood pressure, relax muscles, and boost the immune system.
Don't be so stressed or wound up that you fail to laugh - there is always something funny, even when you hear "Garlic" - "Garlic Bread".
If you don't laugh at that, then fart on the train on your way home, it will also help lower blood pressure and work your internals, but better still, it will certainly make somebody else laugh.
I thought I would add some jokes here, but its more fun watching people make fools out of themselves on sites such as youtube.com
If your feeling a little low or simply want to laugh out loud, then spend a few moments looking at what others are doing with their lives.
Living life without laughter or not living life, its about being on the planet and living a life of no real meaning.
If you can't sum of the effort to laugh, at least try and smile.