I had a basketball coach in high school who was always trying new and experimental things to improve our play. Some were terrible failures. He decided that french fries were the perfect food for us and encouraged us to feast on them before games. He decided that if we all wore the same shoes we would all coalesce into a single-minded machine. These didn't work. We continued to lose as we sluggishly tripped over our identical 20 year-old Converse shoes that were all the rage when he used to play.
One day he told us that he had just read about some recent research and was ready to put it into action. He made us line up at the foul line, close our eyes and imagine shooting perfect foul shots... over and over again. The boys naturally began to giggle, crack jokes and erupt in fits of laughter to which the response was to force us to do hours of wind sprints.
That was my first experience with positive thinking as a strategy for success. He was probably right - a little visualization could positively develop your follow-through. I have since come across other people who seem to take this idea a little too far. Sure, you could visualize yourself in healthy positive situations and you might be able to more easily achieve some of these end results. But does this mean that if I visualize myself winning the lottery, I will have a better chance of winning the lottery? If I visualize Trezeguet scoring on a penalty shot does that mean he will be more likely to score? (apparently not)
It seems to me that positive visualization will only work if you had the power to affect the outcome in the first place. I have the power to improve my free-throw so visualization could work. I have no control over the balls inside a lottery bubble so I could visualize all I want and no different result will come of it.
Some people not only subscribe to this irrational over-exuberance but also to it's converse. I had a business partner who always insisted on thinking about what could go right but he never wanted to hear about what could go wrong. He assumed that even thinking about the negative would bring it about. This is particularly dangerous and doomed to failure. It is in our arsenal of useful evolutionary traits to worry about what could go wrong. Not taken to excess, this is what allows us to avoid terrible outcomes simply by visualizing their possibilities. To ignore those and to only focus on what could go right is just asking to be sucker-punched.
Remember to visualize positively about those things upon which you could have some effect and not on things that you could not possibly have any effect. Also remember to carry an umbrella if you don't want to get wet and if you think it might rain. Bringing the umbrella will not make it rain.