Over on the "Spirituality and Practice" website there's a short article about it. There's also an e-course you can sign up for that will help you work skillfully with disappointment. Here's one suggestion:
Ask yourself, what am I disappointed about? Name your disappointments, and then let them go. Do not keep replaying them over and over in your mind. If necessary, forgive someone who has hurt or disappointed you. Also, forgive yourself for being disappointed. Remember, disappointment is a natural response to difficulty. The key is to not let it rule your life.It's also helpful to use each occasion of disappointment to experience compassion for everyone else in the world who's feeling disappointed at that very minute.