I had a re-occuring nightmare for many years that I learned to conquer by finally learning to lucid dream. The key is to look for something that let's you know your dreaming, something that just can't be. This is easiest to recognize in re-occuring dreams. In my nightmare it was a certain lamp that would never ever work in the dream. I always kept this lamp working, and so when I would try it and it wouldn't work I would know I was having my nightmare again. From that point on I could alter the course of the dream, and manipulate things to go the way I wanted them to.