From this Psychology Today post: Ten Impressive Psychology Studies from 2011
Visualize Success if You Want to Fail
For many years we've heard that visualizing our success is key to attaining it--but an intriguing study conducted in 2011 indicates otherwise. Researcher published in the Journal of Experimental Social Psychology suggests that not only is positive visualization ineffective, it's counterproductive. A practice proffered to help us succeed may do just the opposite.
During the course of four experiments, researchers demonstrated that conjuring positive fantasies of success drains the energy out of ambition. When we imagine having reached what we want, our brains fall for the trick. Instead of mustering more energy to get "there," we inadvertently trigger a relaxation response that mimics how we would feel if we'd actually reached the goal. Physiologically, we slide into our comfy shoes; blood pressure lowers, heart rate decreases, all is well in the success world of our mind's making.
The research also uncovers that the more pressing the need to succeed, the more deflating positive visualization becomes. One of the experiments tested whether water-deprived participants would experience an energy drain from visualizing a glass of icy cold water (a simple but elegant study design) and found that indeed, in even something so basic, the brain responds as if the goal has been reached.
From a "proof is in the pudding" standpoint, the research showed that participants told to visualize attaining goals throughout the course of the week ended up attaining far fewer goals than a control group told they could mull over the week's challenges any way they liked. The positive visualizers also self-reported feeling less energetic than the control group, and physiological tests supported their claim.