You don't visualize yourself taking the steps to accomplish a goal, you visualize yourself and your feelings as if the desired goal has already been accomplished.
Not "I will lose 25 pounds!", but instead "I have lost 25 pounds and I look and feel awesome and am very happy and thankful to have done so!" - A great deal of the secret is replicating the emotional state you'll be in when your given goal is accomplished.
The idea being that, I suppose if you were to remove the supernatural element from it, it somehow gets your subconscious to motivate you to doing whatever it is you want done.
Calling it "positive thinking" really doesn't do the concept justice. Thinking of goals as already accomplished doesn't really fit that definition.