I have been tying flies off and on for a good part of 20 years. I have had some good teachers, good books, and I have learned a lot of things through trial and error. I have learned quite a lot from other forum members. I would like to encourage others to chime in with their own observations. Mine, in no particular order:
1. A small clean head is the sign of a professional.
2. Learning to whip finish by hand was a game changer.
3. If a step looks wrong, unwrap it and do over. Always!
4. Use ceramic bobbins and sharp scissors. You need a reliable vise.
5. A pinch wrap is probably the most important skill you will learn.
6. You don't get faster by moving faster. You get faster by eliminating pauses and knowing where stuff is.
7. Pre-sorting, measuring and working in batches helps a lot in tying faster and more consistently.
8. Proportion, based on the hook as a measuring guide, is how you get correct, consistent flies.
9. Compare your fly to the picture you are trying to emulate..
10. Crappy materials = crappy flies.
11. Sparse flies may not look as good to you, but they catch more fish.
12. Use the smallest threads you can get by with. Use the same brand so that you learn how strong it is and how much strength it takes to break it.
13. If you can tie it on a size 12, you can go down to smaller sizes. Same basic skills. What you can do on a 10 or larger, may not "miniaturize". A 12 requires the same "small fly skills" as a 14-20, IMHO. Big flies require different skills, not less skills.
14. Davie McPhail is not a normal human. Expecting to tie like him is like expecting to pitch like Clayton Kershaw. But you will improve with good instruction and practice.
15. Simple flies are elegant and they work. I.e., Clouser Minnows, Woolly Buggers, Bob's Bangers, PT nymphs, Zebra Midges, Klinkenhammer Specials, Partridge and Orange, and on and on. But they can be deceptive with a lot of subtle differences to get right. I thought I had a Clouser Minnow down pat until I saw Bob Clouser tie one and heard him explain each step.
Fly tying is like sex--you don't have to be good at it to enjoy it, but you should want to get better at it!