Catching big fish is less about "what?" and more about "where?". It may sound cliche, but its true. It's really hard to catch a big fish where there are none, so make sure you are on water with big fish in it. If fishing for trout, focus on lakes with a long feeding season, freshwater shrimp, or just a good reputation for producing big fish. the fact is some places have a larger food source for trout, and just the right chemistry, so fish grow big. most of the major rivers along the I-5 corridor are relatively low in food, which is why the fish evolved to feed in the sea then come back. the most healthy estuary systems will generally produce the larger steelhead and salmon. And make sure you are fishing in the part of that water where they hang out. they eat the same things they always did, just more of it. and bigger fish can fit bigger minnows in their mouths, so fish bigger, but your catch rate will drop. Like troutski said, go deep, sometimes that pays.
Pretty much every trout over 5lbs ive caught, i already knew where he was. big fish like that tend to stick out when on the feed. Especially in a river, but even in a lake.
hope that helps.
matt