When I taught the kids how to shoot, I would occasionally put a snap-cap in the magazine, in some random order with the live rounds. There is nothing that reveals 'flinching' like pulling the trigger on a round that goes click!
I used to do that to my wife. I would hold the pistol aside... pretend I was getting it ready... then hand it to her with an empty chamber.
Even she noticed her flinching and her recoil anticipation that way.