Well, not exactly a secret, but typically if I am in "big fish" mode I am after water that looks like this:
Worth noting is the gravel, not sand or mud. Also worth noting is the steep drop off into deeper water. What you can't see, is that I am standing waist deep, and three steps farther into the river and I might have to swim. Big fish don't like to spend time in shallow water unless they are feeding or spawning, so places like this are great. They have shallows in which to root and feed, but the safety of the deep is really close by. The trouble is you have to spend a lot of time looking, and very little time casting. You move...walk, and look and hope that you happen to be in the right spot when that big carp decides to duck in there for a quick bit to eat. When you get the timing right, and manage to avoid blowing the shot, the results can be pretty good.
Be prepared to give up some numbers...water like the above quickly becomes a game of quality of fish over quantity of fish.