I find that this is something that sometimes sort of just happens to me, when I get aware of something I'm doing better now than, say, a year before.
But, I never really consciously decide to do that, because then I find it just doesn't work. Like the cliché of standing in front of a mirror and saying to yourself "today is gonna be a good day" (Has that ever worked for anybody? Where is this meme coming from?)
It's about making sure you take a moment to appreciate the good things you experience. It's about making an effort to not assume malice when there's a plausible positive or neutral explanation for someones behaviour. It's about the fact that if you smile at people and say good morning, they'll return the favour more often than if you frown and don't. It's about making peace with the things you can't change. It's about not dismissing this stuff as happy-clappy bullshit, but about giving you energy to address the things that actually matter, rather than just going around being angry at the world.
Weather is a good example: If you find yourself constantly annoyed at the weather and it's possible for you to do so, why not go to somewhere with a climate that might better suit you?
I guess my point is, I agree, it's not worth it getting constantly worked up over stuff, but also one shouldn't ignore one's feelings towards certain things for a prolonged time.
The former can have quite an effect on the latter though. To take it to one extreme, if you're never happy you're not going to be satisfied with life. But more realistically if these habits often help you feel happy when otherwise you would've been less happy, then that's going to have a positive impact on how satisfied you are with life.
So I'd say rather than simply saying nice things, one should be conscious of when one is thinking negative thoughts and then try to see where they come from and then what can be done about it. Being honest with yourself is a big part of that and I think that's what many people have a hard time with. And of course: If you're happy, also try to notice and cherish that. (If you look in the discussion I've had with another poster in this comment-thread, we've talked a bit about this too)