Immerse Yourself in the Positive
What I mean by this is to associate with positive people, to read positive books, listen to positive tapes and the like. Every encounter, every positive experience will leave a trace on your heart. We are the sum of the books we have read, the people we have met and the experiences we have had. The key is to realise that you have the ability to choose which books, people or experiences. Don't just drift along, absorbing whatever experiences come your way.
Make a list of the positive people you know, and make plans to spend time with them. If you don't know many positive people, seek them out. Personally I find a lot of such people at toastmasters.
Take at look at your reading pile and maybe re-prioritize your reading. Make sure you focus on the positive. Also, perhaps see if you can read more or try listening to audio books in your car.
Do a time log for a week. Mark each hour as either positive or negative in terms of the experiences you are having. Then review your log and look for ways to increase the amount of positive.
Another trick is to put a rubber band on your wrist. When you catch yourself thinking something less than positive, snap the rubber band to give yourself a sharp pain, and remind you not to think that way.
A year from now, you will be a very different person if you apply this.