my way to always be happy is always looking from the positive perspective of everything that happens in my life. Even the "bad" things can be good if you are positive enough, the most common for me is thinking of the "bad situation" as experience.
A consequence of being like this is never regretting my decisions, as long as I remember them the next time I have to "act"

Also, when I feel really really sad, I usually just act like im happier or more cheerful than usual, because, at least for me, in the end I become surrounded by positivity resulted from my actions

cheering people feels extra cool most of the time