Have the right tools to hand. Pliers/trace blades in our pocket or dealing with lightly-hooked small fish, or cutting the trace if you get a flying hook caught in the net.
Long forceps for most in the mouth unhooking and a pair of mini boltcroppers for emergencies - stapled gob, hook snagged in gill rakes etc.
Best advice of all is go with someone who knows what they're doing. You'll find basically the same advice everywhere - roll them over, kneel astride it, left hand down side of gill plate, lift it up and it opens its mouth.
Then you can go in and turn the hooks out, usually top hook first, bottom second.
Once you get the hand of it, it's usually a fairly quick and painless procedure for both you and the fish.
But you will get cut hands from time to time from the serrations on the back of their gill rakers.
Good advice is take at very least waterproof plasters, a bottle of TCP and some clean tissues with you in a little lunchbox, so you can quickly clean any cuts up, get them covered and carry on fishing. I also carry one of those alcohol hand cleaning gels.
Staggers me how many people don't carry this sort of thing and the number of times I end up giving other anglers antiseptic, plasters etc.
With the risk of Weil's disease on some waters it's worth taking care of yourself.