"Cheer up" is not helpful advice. You cannot force it. Happy moments are not the result of someone deciding to cheer up, but of a happy confluence of circumstances. Too much attention to ones mental states probably is not good either. You might notice that you are happy at a particular time, and usually it is because you are absorbed in doing something, not because you've looked inside, found yourself depressed, and decided to cheer up.
Positive psychology is fine, I guess, as an antidote to the idea that psychology should occupy itself with times human psychology goes wrong. Still, I prefer a more neutral approach, in which emotional states are not the main issue in the first place. What you should be concerned with most is whether you are doing what you are supposed to be doing in life, not your moment-to-moment degree of contentedness.
Philip Lopate wrote an essay "Against Joie de Vivre." To have a satisfying life you don't have to have joie de vivre in some exaggerated sense, making sure that everyone knows how much you are enjoying life.
It is often a good idea, though, to act half-way cheerful around people you meet in supermarket, etc... Not in an exaggerated way, but just to spread a little less dreariness around. If someone responds to your cheerfulness with a smile, then that will make you feel a bit better as well.