To simplify things further, I make nearly all my decisions based on the answers to two questions. First, I ask if saying yes to this choice will make me happy. Second, I ask if I think saying yes is the right thing to do. If both questions are yes, I do it. If not, I don't. Sometimes I'm afraid of what I might be missing when I say no, but that fear diminishes every day because too many good things happen.
This interesting excerpt above is from an article that appeared in Lifehacker yesterday. However, I reckon it is somehow quite incomplete, as you know that there are some things in life that we really should be doing, although we may not be as happy in undertaking them. A simple example would be to do homework, or revise lecture notes for upcoming exams.
I strongly think that righteousness should take precedence over happiness. If you can afford to have both, that would be even better but it is not always the case, isn't it? I would rather choose to be in outright despair of being right, than utterly happy of being wrong.