Somewhere in these threads I wrote up a whole slew of grounding techniques under the title Breathing and Grounding. Maybe there's something you can use there.
One of the editors at the time mentioned putting it somewhere else in here. Maybe you should check with Anthony about it.
Anyway, grounding is one of the best ways to deal with flashbacks. I suspect that's the purpose for slapping yourself. Sorry I can't help more.
Take care, morgan