I've seen some guys do some pretty horrific things for "field dressing" their fish.
Personally, I picked up a medium-sized locking, folding hunting knife with a thick blade that I keep in my belt and a smaller top-handle cooler that I put half a bag of ice in.
If I catch something I want to retain, I use the BACK side of the blade as a swift bludgeon to the fish's forehead. This will render them unconscious with a single tap(it really doesn't take much). You don't need to smash their head with a rock or bat, they have small brains that are very close to the surface of their head and you only need them unconscious for the next 10-15 seconds. Once that's done, I use the knife to cut ALL of the gills completely through and bleed the fish out upside down with it'd head under moving water. You want to swish it back and forth to keep the blood from clotting so you get a full bleed.
Once you cut the gills, the blood pressure in the fish's brain will plummet and it will stay unconscious. Any movement is purely reflexive at this point so don't worry if you drop it, it won't swim away.
Once the water is clear around the fish where you're swishing, flip it over and do the rest of your field dressing. There might be a bit more blood at this point but once you've gutted it, rinse it off thoroughly and put it in the cooler.
Usually, no more blood should show up, the cooler should stay clean and usable for the whole day and the fish temperature should stay low enough to prevent any "stink" forming.