Land your fish quickly. Don't play it to exhaustion. Small fish can be aquaplaned in close by keeping the head high in the current.
Leave your fish in the water, if possible. Net your catch only if necessary.
If you are wade fishing, simply bring the fish to your side. Then, reach down with your forceps, grasp the hook and release it.
If you use a net, the net bag should be made of soft material (many are not and scrape the surface of the fish). Leave the fish in the water in the net and release the hook. If the hook is in the fish's gills or throat, snip the tippet and release the fish.
If you need the fish calm, slide your hand carefully under the fish and turn it upside down. In the upside down position a fish seldom struggles.
If you must handle a fish, wet your hands first to avoid removing the protective slime on the skin of the fish. Never squeeze a fish around the middle or touch its gills. Internal injuries and bleeding from too much finger pressure are common causes of death in released fish. Never let a fish flop around on the rocks or in the boat.
Most fish die because they have been released before they have had time to recover from the fight. No fish should ever be thrown back into the water. To revive a fish prior to release, hold it in the water just above the tail. Keep it upright facing into the current. Push the fish forward and back slowly until the fish is ready to swim strongly on its own. This may take a few minutes. It does not hurt to revive a fish a bit longer than you feel is necessary. When fish are released too quickly, they will often swim out of sight, lose their balance, turn on their side and die.
If possible, release your fish in quiet water and allow the fish to swim into the current at its leisure.