Minimize Air Exposure
Just like humans, fish experience exercise-induced stress causing them to tire and have diminished muscle function. In order to recover from being caught, fish need to be in the water so they can breathe and pump oxygen into their system.
Holding a fish out of the water prevents recovery and can lead to death if done for too long. Even shorter duration (as little as 10-20 seconds for some species) can have serious effects both on short-term and long-term fish health.
You can reduce these health effects by keeping a fish’s mouth and gills fully submerged in the water as much as possible during handling.
Eliminate Contact With Dry Surfaces
Fish have a layer of protective mucus (“slime”) that protects them from diseases. Contact with dry surfaces, whether they are hands, grass, boat bottoms, etc., can remove this protective slime and make fish more 
susceptible to diseases. Additionally, if placed on a dry surface, there is an increased likelihood that a fish could injure itself by thrashing around on lakeside rocks or the bottom of the boat. You should try to do the following:
- Land fish in the water.
- Wet your hands prior to handling fish.
- Hold fish in or slightly above the water, away from dry or hard surfaces.
Reduce Handling
Generally the less a fish is handled, the better, so you should prepare in advance and take extra steps to minimize handling time.
- Land fish quickly without playing it to exhaustion.
- Hold fish in net or over water.
The fish will be unharmed if you drop it. - Fish barbless hooks and carry a hook removal device.
- Use rubber nets.
- Photograph fish wet.
- Grip fish carefully.
Carefully revive fish. In stillwater move the fish in a figure eight so water goes through the mouth and over the gills, not back and forth!
For more tips and best practices, go to KEEPEMWET FISHING.