Nearly everybody is bad at this. Circle hooks can do a lot to improve the situation (though they have their disadvantages). With circle hooks, you simply start reeling in as fast as you can when you feel the weight of the fish or feel the fish moving off with the bait. Of course, this doesn't apply to jig fishing, where you set the hook ASAP. Regardless of method, however, I make certain my hooks are sharp so that fish hook themselves or only light pressure is necessary.
Keep constant tension on the fish, and you will lose few of them. There are exceptions to this, but nearly all the time, it works. Don't use your reel as a winch: reel in only as you lower your rod tip. A medium fiberglass rod with nylon mono line excels at this (keeping constant tension), while the opposite is true: a fish is can be much more difficult to keep on with a graphite rod and a no stretch line.
Buy a net, or a gaff, appropriate to the spot you are fishing. I've lost plenty of good fish because I came unprepared. Remember, if you lose a fish due to equipment failure, it's probably a big fish. Only the big ones get away.