One of the hardest things in life is to change your way of thinking. As hard as it might be to change your actions or what you say, it's infinitely easier to change those things than to change what goes on in the confines of your own mind. Thoughts come almost inadvertently to the front of your mind, and sometimes it feels like you can't do anything about it. In the bible it says that thinking a thought has as much impact and consequence as actually doing the action- a sinful thought is just as bad as a sinful action (VERSE). Well that's kind of frightening, no? I mean, normally we think that our thoughts won't hurt anyone- it's not like anyone can hear or see them anyway, right? WRONG. Firstly, God can hear all of our thoughts before we even think them, and it pains him to see us thinking a lot of the things we envision. Try to remember all of the thoughts you've had in the past few hours. Next, separate them into separate groups based on their nature; were they selfish, selfless, vengeful, amiable, or inconsequential? Look at the list. Each thought in the negative columns counts as a sin. And that's just recently. What about all the thoughts you've ever had? Good days, bad days, everything in between, it all counts. It's a scary thought. What are we supposed to do with something like that?
CHANGE IT. As hard as it is, change what you're thinking, because not only can God see what you're thinking (which is rather important), everyone else will eventually be able to also. Whatever you're thinking eventually becomes something you say, or do, or imply. Your thoughts dictate your words and your actions, whether you like it or not. What you're thinking is really who you are- what you say and do is who you want other people to think you are. Eventually the real you- your mind- is gonna come out in some way or another.
Examine your thoughts again. Don't like what you find? Yeah, me neither. As good of a person as you might think you are, you're a sinner. Everyone is, unless you happen to be the Son of God, and there's only one of Him. So as hard as it is, don't think the bad stuff. Keep your thoughts as pure as filtered drinking water. Purer, even. Someone makes you mad, pray FOR him or her instead of envisioning him or her failing miserably at his or her next soccer game, or embarrassing him- or herself during his or her speech.
It's a high goal to aim for, I know it, and in the beginning it's gonna be really, really hard. But in the end you might just find that thinking happy good-luck thoughts for other people might actually make you feel happy yourself. Gee, what a concept. :)