Research shows that when you feel sad and you force yourself to smile then you feel happier. I’ve been doing this way before I knew about any research.
When nothing is going your way. Smile.
When you have a bad day, smile.
When you think you are born unlucky. Smile.
These days I even smile in my sleep and I sometimes laugh at the ridiculousness. Though I will warn you that this may or may not be slightly creepy if someone sees you in the act.