|The people you associate with have the biggest influence.|
For example, tonight, you plan on watching a movie. You can choose any movie you want. How will you choose? If you are controlling your environment so it has a positive effect on your attitude, you'll choose movies that uplift you, inspire you, enrich you, or make you laugh. You will tend to avoid movies that overstimulate your adrenal glands. You'll avoid movies that give you nightmares or make you feel anxious.
When you watch a movie, it effects your emotions. Your body produces chemicals that circulate in your blood for awhile. If you watch a suspenseful movie, it is designed to produce anxiety in you. Those stress chemicals are still circulating in your blood even after the movie is over, and those feelings effect your perspective on the world, at least for a little while. And images from the movie will spontaneously come to mind for many hours afterwards. That's bad enough, but do it often enough and it can actually change your worldview.
The same holds true for music. Listen to music that has a positive effect on your attitude. Don't listen to what you merely like. What you like is only one criteria. Use more criteria than that to make your decisions. Be more selective. Increase your standards. Do you eat nothing but cookies and cake simply because you like them? No. You have more criteria for eating food than that. You want your food to taste good, but you also want it to be good for your health. Listen to music you like, but that also has a positive effect on your feelings, and has positive words. Read and listen to books that leave you feeling stronger or happier or more motivated.
Forget about newspapers and television news. For more on that, read Profit of Doom.
The movies you watch, the music you listen to, the friends you hang out with — these are only a few of the elements of your environment you are capable of influencing. The list is a long one. What about the smell of your environment? What about how it looks? How clean it is? What decorates the walls? You could post a picture of a national tragedy or an inspirational saying, or anything in between, and it will have an affect on you. What about the desktop of your computer? Keep looking and you'll find many areas of your environment that you can control that will influence you.
To some degree, you are the effect of your environment. But it goes both ways. Your environment can also, to some degree, be changed by your actions. So take actions that make your environment create a good effect on you. Use what power you have.