I think getting the fly down far enough is the biggest single element for success. In that vein, I know one fisherman who swears that it's better to use split shot 18 to 24 inches above the fly than to weight the fly itself when tying them. I however, weight the flys when tying. Either way, get the fly down there to the fish. If you are not losing some occasional flies to the rocks you are not down deep enough.