Martin Seligman, Ph.D. is the founder of the positive psychology movement, and author of Learned Optimism: How to Change Your Mind and Your Life and Authentic Happiness: Using the New Positive Psychology to Realize Your Potential for Lasting Fulfillment. These are books I recommend highly.
In this video, he describes an exercise where you examine what has gone right in your life that day, rather than dwelling on what has gone wrong. Many studies show that focusing on what you have to be grateful for increases your overall mood and well being. It trains the subconscious to scan for things to be grateful for and creates a sense of optimism.
In the exercise, you think of constructive things that have happened, and what caused them to occur. This allows you to focus on what’s positive. And energy flows where attention goes. So this is an easy technique to begin boosting your positive thoughts.