If it is done well, positive thinking works well. Certain principles can help you do it effectively. For example, consider positive thinking to be a kind of self-coaching, and always coach toward a purpose.
Another way is to give yourself advice. It helps to imagine what a wise person would tell you.
Another good way to think positive is to use anti-negativity. To find out how to use these principles, and to learn more, check out this article: Does Positive Thinking Work?