I've been asking the same question of my current (and past) therapists.
His cure (like loads of other white coats) "Rewrite your nightmare for a happy ending".
Na, never works.
Only two things help me.
My wife for nightmares and my dog (and maybe other shoppers) for flashbacks.
Wife: All it takes is a simple touch and I'm awake.
Then comes my tears, she holds my hand and tells me "it's alright".
Once I hear those two words, I can go back to sleep.
Doesn't prevent the nightmares or moderate them, but it is a gentle way out of them.
Dogs Turn.
Flashbacks happen without warning nowadays and I just switch off. Middle of the road, shopping, taking a shower, walking the dog. Lights out, I'm there reliving things.
Dog instantly stops me roving. Barging me into the kerb to keep me safe sort of thing.
Then I get "the wash!" Instant wake up. In shops if anyone puts their hand on me when i'm switched off, I wake up. Confused but now safe.
Now my nastiness comes out. I'm not alone with the lack of a therapists magic cure.
Often it's the same song from therapists. Repeated word perfect, and straight out of the textbook. Do this son and you'll be OK.
Nope, and I guess I'm one of them statistics where 75% of people don't win.
Want to see the dog?
Well big enough to push me out of harms way.