Here's a really nice, short piece about one man's fly tying evolution by Dave Karczynski, I found posted on Midcurrent. He clearly has thought out the process, and along with an artistic bent, he offers some good points that I think every fly fisherman/fly tyer can apply to their own approach.
Click here, and enjoy: Flies of Innocence and Experience
The only thing I might do differently than Mr. Karczynski, would be my approach to having just one "hot fly of the day" left. He opines that he would fish the fly tentatively, for fear of losing it. Conversely, it wouldn't matter to me whether I have ten, or only one, of the fly the trout are taking like candy. I would fish that one fly with the same zeal and abandon I would if I had a boatload of them. I'd rather give myself every opportunity to catch fish with that one fly, then end the day with that fly still on the end of my tippet, and lost chances in my head. Besides, if you do lose the fly, it's just another opportunity to learn something.