Meditation is one of the best practices I know for flashbacks (have them myself). My favorite meditation for this is to sit somewhere quiet, picture myself sitting on a rock in the middle of a dark cave with water surrounding it. I visualize water dripping off a stalactite and into the pond. Picturing that single drip of water repeating at a set interval helps to calm and focus my mind.
If this does not work for you, there are tons of other similar meditations out there. However, I have found doing a simple deep breathing or attempting to not focus on anything and just "relax the mind" triggers my flashbacks. This is most likely due to the mind trying to work out the issues causing them.
Therapy helps some if you can find a good therapist that specializes in that area. Utilizing sensory deprivation as a means of facing the fears, can also be benificial. I would not suggest doing this without either a trained therapist or a trustworthy friend. It can be a bit upsetting and depending on how bad your ptsd is it could make you feel worse at first. Definitely worth it in the long run though. Hope you find something that works for you :)