As well as watching and learning from others as they fish. I am lucky enough to not so much host, as be tolerated and entertained by, some exceptional casters and fishers from the States annually. I have learned more in the past few years from watching these guys than they have forgotten over a lifetime of perfecting their craft.
For me personally though, it is observation of local waters, getting to know each pool and run on a familiar beat, learning where the rocks (both protruding and hidden from view) lie, how the trout behave around these obstructions to the water flow, at what times they are feeding and what they are feeding on.
Often I will walk upstream on the high bank, not the ideal position for a fisherman to take, but it does allow a better sight picture into the water. As I spook each trout from its lie, I will make a mental note of where it is (often using a bank feature, flood debris or a tree as a reference) so I can return to the same spot from the other lower side of the river...... hopefully unseen by the trout that has resumed feeding.
You can use this high bank" approach with a bit more stealth (not "sky-lined" and silhouetted, thus spooking the trout) to study the feeing trout before planning your attack from the other side, taking note of the way he is feeding and forming a mental note of just how your fly will need to be presented to fall into his feeding lane. You can also generally make out just where in the water column he is feeding, thus helping with fly selection.
This really applies to fishing in Southern NZ and sight fishing to a particular trout, which makes up maybe 80% of fly fishing here. It may not be as relevant in other waters where fish can be induced into rising or striking a drifting/swinging fly........ but sometimes it is still fun to do!
