I have found that once I am able to make meaning of the nightmares (not to be confused with flashbacks, although sometimes they intertwine), they tend to dissipate for me. Sometimes the meaning has very little to do with the bad stuff, and it's really valuable to explore in therapy.

As far as flashbacks go, I relate to the idea that it comes and goes....There will be months that go by when I haven't had a single one....and then all of a sudden, I'm struck with them every night and avoid sleeping. During those times, I try to pay attention to what else is going on in my life to see what may be triggering it....or, if it's a matter of just being reminded by talking about it in therapy, grounding myself in the present can sometimes help.