Dry fire.
Grip exercises.
One of my favorite things to do when I'm at a public range(I've posted this before) is while observing safety rules, I like to do either 25-30 jumping jacks and/or 25-30 push ups, then while my pistol or shotgun is on the table, grab it and proceed to try to put rounds accurately on target. I like to think this helps shooting under stress as not only is my heart beat a little faster than resting, but it helps get a little blood flowing too. My shooting has gotten slightly better since I began doing this.
I also workout a lot and I truly believe that keeping yourself in some semblance of physical shape will greatly help you if SHTF because you will at the very least know how to control your body with an increased heart rate and know what it feels like to have your heart pounding. Not to mention the health benefits.
