One common failure I see of many anglers I guide is when walking up the river with fly in hand, or coils of line held 'for quick casts'. Unfortunately, unless you spend a lot of river time and monitor your line well, when a fish is spotted and needs a fly in front pronto the angler is usually fumbling around, giant mess in hand with line tangled to the hilt.
Guess what? We don't get a chance to make that cast... Another chance missed...
I always instruct clients to attach the fly to the rod and wind up tight, even if only stalking a short distance up a pool. This results in less hassle, fewer messes and more flies in front of fish during those precious early moments when you locate that trout. With fly attached to a guide up the rod, leader wrapped behind the reel and all going well, the below 'tap technique' gets you on the river, and fly in front of fish in no time at all... Its also a great way to quickly present a fly to those trout you find yourself suddenly standing next to, a rod length distant that you never had a chance at... Well guess what, you did. You just weren't ready.
Because its the many little things, executed well that make a big difference to success, and why 10% of anglers catch 90% of the fish... And 1% even more so.