Deep Breathing Technique
One quick-to-learn technique is based on deep breathing.
Close your eyes, and think of a distressing memory.
Rate the level of stress in that memory on a scale of 10, with 1 being the least strong feeling, and 10 being the greatest possible.
Then inhale deeply, and very slowly exhale to the count of four. "One thousand one, one thousand two, one thousand three, one thousand four."
Inhale deeply and repeat. It is very important to exhale very slowly, and not just to breathe deeply in a normal way.
Do that for a couple of minutes, and then think of the distressing event and rate the remaining feeling. If that rating is anywhere above zero, then repeat until it is eventually zero.