According to one study, it's writing things down - not talking about it. Though I suppose writing about it here would qualify:

Those who had spent time talking about their traumatic event thought that the chat had been helpful. However, the questionnaire results told a very different story. In reality, the chat had had no significant impact at all. Participants thought that it was beneficial to share their negative emotional experiences, but in terms of the difference it made in how well they were coping, they might just as well have been chatting about a typical day.

So, if talking about negative experiences to a sympathetic but untrained individual is a waste of time, what can be done to help ease the pain of the past? As we saw at the start of this section, trying to suppress negative thoughts can be just as unhelpful. Instead, one option involves “expressive writing.” In several studies, participants who have experienced a traumatic event have been encouraged to spend just a few minutes each day writing a diary-type account of their deepest thoughts and feelings about it... participants experienced a remarkable boost in their psychological and physical well-being, including a reduction in health problems and an increase in self-esteem and happiness.