If I could only give one piece of advice in the "How do I become a better fly tyer?" department, the answer would be easy. "Never tie just one of anything."
What I really mean to say is, "Tie a lot of every pattern you tie." But nobody wants to hear that, since it sounds like work. So I compromise and figure that even two or three is better than just "one in a row."
There are two fundamental reasons for this. The first has to do with practice, and the second has to do with organization. For practice, it's simple. You just get better with every fly you tie. This is true for beginners, and to a lesser extent, advanced tiers. (This is actually a corollary to my "The Next One is always Better" rule.) I find that when tying normal fishing flies, it takes me about a dozen until I feel like I really understand the construction of the fly. It takes me a few sessions of a dozen before I have it memorized, and can tie it by heart. Once you have the pattern memorized you can start working on making the proportion accurate, and economizing your motions. So it's all about repetition. Now I'm not saying that you need to tie three dozen Gold Ribbed Hairs Ears each time you sit down. But I am saying that the second one will be better than the first one, and so on. The more you can extend that, the better.
And organization, it's all about getting your stuff together. Compare the amount of time that you spend tying a single fly with the amount of time you spend chasing down all the materials, pulling them out of their wrappers, cutting off segments, etc. It's one thing if you have all your materials in one box, but for those of us who have taken over a whole room, well, it can be painful.
I'm amazed at some of my students who sit down, tie an Adams, put everything away, tie a Cahill, put everything away, etc. (I call it the "Fly Tying Kit Syndrome.") To this end lies madness. You're so much ahead of the game to tie three Adams's the first night, three Cahills the second, and so on. You'll produce more, and you'll be a better tyer in the process.
You did buy Production Fly Tying by A.K. Best, didn't you? It's all in there.
Finally, once you get past the "multiples" stage, you can start working on tying flies in stages. When I say I never tie just one of everything, I mean everything. Even when I'm tying complex full dress Atlantic salmon flies, I'll still build up three or four wings at a time (it takes so much longer to find the materials, hold them, grab the scissors, than to cut - that I always cut several segments). I put extra wings in small photo albums, where they stay flat, safe, and easy to see. I tie up the bodies the same way. As long as the materials are laid out, tie a couple more.
When I tie Carrie Stevens-style Rangeley streamer patterns, I usually build up the wings first - using the Carrie method - and then assemble them to the bodies later. As you can see in the picture above, I take a sheet of black Plexiglas (easier to see) and lay out my left and right sides for each fly. I glue them up, and then hold them in place with something like the tweezers or the dice shown above. I do one feather at a time, working my way across the board until I've done my last lap with jungle cock. I then let them sit and come back the next day, when it takes just a couple of minutes to tie the preassembled wings on the pre-tied bodies. It's faster, and somehow, I find it more satisfying to whip out larger quantities like this. So, starting today, Never Tie Just One. If you've got enough time to sit down to tie one, you can tie two. And so on...