I have been trying to do this for years. Me and a few friends of mine even went as far as to declare a “Bitch Free Day” on days we knew were not going to be good. It helps to have others reminding you to stay positive.
It is a hard habit to break. I used to look at it as being confident in myself and what I had to say, but now I realize others really dont want to hear it. It is better to be positive or not to say nothing at all (kind of feel like my mother saying that). Anyway, you get better response from other people which in turn will make you a happier person as well.