A lot of anglers leave the river after a nymphing session wondering why they dropped so many fish. It’s the hook set, baby! This is such a simple principle. Adhering to it will result in a noticeable increase in your catch rate. Check out this diagram:
A proper nymphing hook set goes downstream, into the mouth and the mass of the fish.

If you accept the proposition — and I feel strongly about this — that most fish are won and lost at hook set — a good set is critical to nymphing success. Picture your fly moving downstream, a few inches off the bottom. The trout is facing upstream, sees the nymph, and decides to eat. You detect the strike (look for a reason to set the hook on every drift) and set the hook. Don’t set upstream. Doing so essentially takes the fly away from a fish that has said “yes” to your offering. Instead, drive the hook point home into the fish’s mouth — downstream — using the mass of the fish against itself. What if you’re indicator nymphing and your drift has the fly 30 feet below you? On the take, sweep set off to one side.
Do this every time and you’ll be netting a lot more fish. And of course, you’re constantly checking your hook points to make sure they’re sticky sharp…right?