The first secret of happiness is that it really isn't a secret at all. It is something that you already know how to do. If you are not already a happy person, for whatever reason you have chosen either consciously or subconsciously not to make happiness as high of a priority as it should be.
So what is it that one needs to do differently in order to be happy? Think back to a time when you did something really nice and unexpected for another person, asking nothing in return. How did that make you feel? Now think back to a time when you acted completely selfishly at someone else's expense? And how did that make you feel?
When we are lost in the constant flow of discursive thoughts, allowing them to run amok, we become less aware of our subtle physical, mental, and emotional sensations. However, if we pay attention to the sensations we encounter when we act selfishly or in a harmful way toward another, we find that we experience one or more unpleasant sensations, such as increased heart rate, rapid or uncomfortable breathing, a slight tightness in the chest, sweaty palms or a queasy feeling in the gut. We may experience unpleasant emotions such as regret, or guilt.
Whenever we act selflessly, especially if we perform some random act of kindness, we find a completely different set of sensations. On the subtle side we experience a calm, peaceful, contentedness. On the less subtle side we experience a tremendous sense of happiness and satisfaction. The bigger the act, the bigger the reward.
This little secret isn't really such a secret anymore. There are movies like "Pay it Forward", TV shows, books, magazines, and even websites that promote acts of kindness. There is a "Random Act of Kindness" day. There are websites like www.randomactsofkindness.org. Another site, www.helpothers.org actively supports and promotes random acts of kindness through stories, ideas and the use of "Smile Cards" that are available free on the website and can be used to keep the random acts moving in a pass it along fashion.
Following this path would then imply that the ultimate form of happiness can be obtained through altruistic pursuits. It isn't hard to see. Altruistic people are always happy, they know the secret. Conversely, look at any greedy, selfish person. Are they ever truly happy?
So then, what is the real secret of happiness? The real secret is that happiness is a matter of choice. You can choose at any time to act and behave in ways that promote happiness in your self as well as in others. The only thing that gets in the way, for most people is ego and thought patterns that promote selfish, greedy behavior. Make a conscious choice to live selflessly, pay attention to your thought patterns, and work at doing what makes you and others happy while avoiding that which makes you and others unhappy, and the happiness rewards will always follow.