Practicing a good meditation technique should do it, in my not-so-humble opinion.
ask yourself, "is this situation worth getting myself upset about? it is really worth all my effort?"
Find something that makes you happy. Personally I like to write Diaries, poetry, short stories to get my mind of the things that get me down, or if I've just had a bad day Music is a great way to get a boost of happiness.
Make urself busy and do things u like dress ur self up, eat ur favourite food :D
Sometimes walking away from a tense environment can help to clear your mind. Also writing down your feelings is another way to release emotions.
Its really works for me too. I feel a sense of relief. But sometimes old feelings do creep up, and I do feel upset again. I guess its about finding a balance. Its healthy to be upset about things, but its gets unhealthy when it overrides your life and your constantly feeling down. Anyway, wishing you all the best.