The physical act of smiling lifts a person's mood. Thich Nhat Hanh, a Tibetan monk and teacher, says that people can't smile without being present in the moment. His book on dealing with anger, called Anger: Wisdom for Cooling the Flames is a helpful resource.
Another resource is the book Never Act Your Age by Dale L. Anderson, MD. Anderson states that "too often, social, academic, and scientific wisdom teaches that if we eliminate the negative we can accentuate the positive. This conventional thinking seems to miss a vital piece in the puzzle of life. It's backwards. In reality, accentuating the positive eliminates the negative."
Here is a smiling exercise that may help you shift your mood:
As you breathe, say the following statements in your mind. Repeat the exercise for several minutes.