Positive thinking is fine if you use it realistically. If you focus on your goals, you do stand a better chance of achieving them because you keep reminding yourself of what you're trying to achieve; however, you also won't achieve them by doing nothing but focus on them.

You know me. I'm a great fan of NLP, which is pretty much the ultimate positive thinking technique. It's a useful tool, but I don't let myself think it's the answer to everything, and therefore I can use it without fear. :-)