Remembering joy

"It is an art - and it belongs to the art of living of Epicurus, the most sublime and most reflective of the hedonists - to make the remembrance of past joys into a source of present comfort in situations of suffering."

- Robert Spaemann, Happiness and Benevolence (trans. Jeremiah Alberg, S.J.; University of Notre Dame, 2000 [1989]), 34-35.

The alternative, of course, is that memories of past joys can make present suffering worse through the slow poison of nostalgia. How is it possible to avoid this? In what does this art of joyful memory of which Spaemann speaks consist? How can we remember with joy that which we no longer enjoy?