I find it interesting that most, if not all, of the responses so far have to do with casting, admittedly, an important aspect of the sport. However, my most memorable lesson, including all the casting lessons I had, came one day when my mentor and I went to the river and to my surprise, he said leave your rod on the bank, as he did with his. We then began picking up rocks, turning them over, looking underneath them and plucking bugs (nymphs) from the river and catching flying insects. We spent a good deal of time examining these critters until I had an understanding of mayflies, caddisflies, stoneflies, dragon and damsel flies as the food trout eat, that I was trying to imitate, how to identify them and their behavior. That profound experience in one afternoon gave me a fundamental understanding of what it was I was trying to do. I still remember that little brook in Connecticut fifty years later, could find it to this day but I honestly don't remember if we caught any fish that afternoon. Of course, this same mentor and I spend many hours on a grass tennis court fishing for grass trout until I had an equal understanding of presentation (beyond just casting) and many hours during the winter huddled over tying vises putting it all together so that now it all (occasionally) works and I catch a fish. We're friends to this day even though life and circumstances have physically separated us by half a continent.