Martin Seligman in delancyplace:
“Here's a brief exercise that will raise your well-being and lower your depression: The gratitude visit. Close your eyes. Call up the face of someone still alive who years ago did something or said something that changed your life for the better. Someone who you never properly thanked; someone you could meet face-to-face next week. Got a face? Gratitude can make your life happier and more satisfying. When we feel gratitude, we benefit from the pleasant memory of a positive event in our life. Also, when we express our gratitude to others, we strengthen our relationship with them. But sometimes our thank you is said so casually or quickly that it is nearly meaningless. … Your task is to write a letter of gratitude to this individual and deliver it in person. The letter should be concrete and about three hundred words: be specific about what she did for you and how it affected your life. Let her know what you are doing now, and mention how you often remember what she did. Make it sing! Once you have written the testimonial, call the person and tell her you'd like to visit [him or] her, but be vague about the purpose of the meeting; this exercise is much more fun when it is a surprise. When you meet her, take your time reading your letter.
“You will be happier and less depressed one month from now. …
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