There's different ways to improve your mind (thsi area is my specialty). Some are more high-level and some are lower-level.
A good low-level technique is to read more books. And not just any books, specific books on psychology and such topics. Blink, The Tipping Point, The Wisdom of Crowds, Stumbling On Happiness, Mean Genes, Influence (big recommendation here) and the like. Assimilate more nad more new ideas and build more
mental models.
Another avenue of study you might be interested in is learning more about how we function. Most people think we either work off of our minds or our emotions, but that's really not the case. We have highly influenced by biases or states or habits that we're not even aware of, most of the time. We routinely engage in fallacies and mistakes and errors that if you learn about them, you'd go, "WTF? How could I not have noticed that?"
Try to read and learn something new every day. Look up logical fallacies at wikipedia. Here's more info about cognitive biases:
Cognitive bias - Wikipedia, the free encyclopedia List of cognitive biases - Wikipedia, the free encyclopedia
In terms of changing your beliefs, this is very important, because we are creatures of habits (beliefs being thought habits, if you will), so its best to get the right habits in and then put it on cruise. You can do a 30 day trial and visualization and affirmations like you said. Figure out how a person with the new belief would react in a situation and then react like that and you can turn your beliefs around. And like regular habits, you don't just get rid of those habits, you replace them.
Good luck!