I was getting this about a couple of specific events so I tried journaling about it: Pennebaker method, (http://en.wikipedia.org/wiki/Writing_therapy) - basically, 20 minutes non-stop writing, focusing on both thoughts and feelings about what happened, for three or four consecutive days - I find three is enough. If it's working, the narrative will change. There was a noticeable reduction in the flashbacks and also a lift in mood. Probably doesn't work for everyone, though.