+1 to Mosca and perigrenes.
Think real estate - location, location, location and as we all know in this real estate market; timing, timing, timing.
Think also biology. Any living thing must optimize energy in (food) - energy out (catching food). The larger the organism, the more energy it takes for it to feed and therefore, it must either get more calories per food item and/or it must be more efficient in feeding. Both occur as fish get larger.
For a large fish to survive it must be an efficient feeder. That means it must seek a location where it can feed very often on smaller items like insect hatches but expend very little energy. Or it must feed when it can feed on large items, like at night from an ambush position. So look for prime feeding lies. I wrote a FAQ on reading water
that explains "lies".
Look for hydraulic cushions like in front of a boulder, or areas of slow flow like next to a bank; you get the idea. Within this spectrum of prime feeding lies, the largest fish will be in the most difficult lies for presenting the fly.
For example tight to the bank under an alder branch. Or behind a low lying branch in the water with other branches just above it. They will feed imperceptibly, with little noise and surface disturbance unless they feed at night. You likely will NOT see them feed during the day unless you are looking for the slight dimple of the subtle rise.
Also fish in fertile rivers. Newbies never consider that the more fertile a river is the more fish there will be. The more fish there are, the most lies will be filled and the fish will be larger.
Top illustration is an infertile freestone river and lower illustration is a fertile spring creek or tailwater fishery. More fish also means more big fish. The better the lie, the larger the fish so for any given lie, the fish will be larger in a fertile river.
So location, location, location - choose a fertile river and the best feeding lies, then OBSERVATION! Find the hidden large fish. You may have heard the adage, "Be the heron". That means spend some time looking and observing before casting.
Then comes timing. Fish when they are feeding. During hatches or spinner falls or at night. In tail waters, at the correct water level.
Finally PRESENTATION. Hone your skills so you know what fly to use and how and where to present it.
Fishing is like selling real estate - Location, Timing, Observation, Presentation. Where is the property, when will it sell, find a likely customer, and finally close the deal.
Search Google for
"Gary Borger, Marker Fly"
and go to the first search item to read about how this is done.