Evict negative thoughts. The first thing I have to do is get rid of my negative thoughts. I used to try to alter them, but I find it's better to just get rid of them altogether. I simply think about something else. Even if the things I worry about will actually happen, it's not the end of the world. Life does go on.
Use a mantra. If you hear something often enough, you believe it. As a child, I heard daily that I was "stupid, ugly, and crazy". Those are the exact words that were used and is exactly why they are banned from my home. I use the same method that tore me down to build me up. I don't sit around complimenting myself all day. But, I do often start the day with a mantra for success. I repeat it until I believe it.
Give yourself a positive environment. I don't mean to surround yourself with a bunch of useless material items. There are people in your life that are emotional vampires. They quite literally feel good after they've done what they can to make you feel bad. But, you let them make you feel bad. Stay away from these people because no matter how hard you try, they will never care for you as much as you care for them. That's not a reflection on you either. Aside from people, bring some color and sunlight into your life.