Adopt a brisk and cheerful 'no big deal' attitude to the things that go wrong in day to day life - forgotten homework, being told off by a teacher, etc. Obviously, you'd take a different approach to serious things like bullying.
Encourage them (assuming we're talking school age children) to come up with solutions and strategies in advance - e.g. the bus home is cancelled, do you wait for the next one, get another bus that drops you a bit further away from home, and walk from there, or what.
Set an example of generally being positive rather than negative about life.
Basically, encourage a mindset that things will go wrong sometimes, but they can deal with things, that the world won't end if something goes wrong.
And try to ensure they're getting enough sleep and ecercise, as lack of those can have a major impact on mood.
Don't be, or encourage them to be, like some posters on MN who are permanently 'fuming' or 'livid' about things.