This simple facial expression is a wonderful way to lift your mood. If you're not in the habit of smiling often, then fake it! The brain doesn't differentiate between real or fake. Your brain will, however, realise that there are changing facial muscles and this will release endorphins. This, in turn, will have a positive effect on your wellbeing. Smiling also makes you more approachable and appealing.
So what, now what? Find things to smile about: watch funny films or listen to music that makes you smile. Think happy thoughts and smile at people at the till points as you pay for shopping. Remember that a smile releases endorphins.