Support from the belly!

Belly support keeps the fish from squirming and flopping too. I don't know if it's pressure on the swim bladder or what but they stay still 99% of the time for me.

 

Also a fish is more likely to start flailing if it can see that it's close to the water. Like you can have it on shore taking a photo no problem, but once you get close to the water it will try and make a jump for it.

 

I get caught off guard sometimes too. If I get distracted I think my grip may weaken just a little. The fish senses this and shakes loose.

Push them out to the camera, I want to see the fish, not your ugly mug! ? 

I bank fish exclusively and still use a flip phone. My best option is to lay the fish on the grass next to my rod for size reference. I can't imagine less than one minute on cool grass is that harmful to their slime coat.