I really like what Henry Nouwen writes about the discipline of gratitude:

Gratitude … goes beyond the “mine” and “thine” and claims the truth that all of life is a pure gift. In the past I always thought of gratitude as a spontaneous response to the awareness of gifts received, but now I realize that gratitude can also be lived as a discipline. The discipline of gratitude is the explicit effort to acknowledge that all I am and have is given to me as a gift of love, a gift to be celebrated with joy.

Gratitude as a discipline involves a conscious choice. I can choose to be grateful even when my emotions and feelings are still steeped in hurt and resentment. It is amazing how many occasions present themselves in which I can choose gratitude instead of a complaint…The choice for gratitude rarely comes without some real effort. But each time I make it, the next choice is a little easier, a little freer, a little less self-conscious.