Most fly anglers devote a lot of time and energy into honing their casts, as they should. The cast is what allows you to play the game in the first place. Without a good fly delivery, your chances of hooking a fish are minimal.
But the effort and focus shouldn't end as soon as that fly hits the water. In fact, I think that's when the real attention to detail should begin. How you drift your fly (presentation) is as important as how you cast your fly, and mending the line (moving the fly line upstream or downstream of your flies, depending on the currents, in order to eliminate drag) is the key to making that happen.
Most anglers I encounter on guide trips have a pretty good understanding of the importance of mending. But 90 percent of us (myself included) sometimes get in the habit of making sloppy mends; chucking the line upstream, whipping the rod, and so forth. A sloppy mend can be as damaging to your drift as no mend at all.
A smart angler will actually practice mending just like they practice casting. So what's a good mend? It is simply moving the entire line from one spot on the water to another, without moving your flies (or strike indicator if you are nymph fishing) at all. You can't do this by flinging the line, or flicking the tip of your rod from waist level. You should realize that fly rods are long for a reason. Use that length to lift and place. Nothing more. Lift the rod, sweep the line, and set it back down. A great mend is all about mechanics, and requires little effort at all.
With time and practice, you'll learn exactly when to mend — when to move the line upstream (when you are casting across heavy current), when to mend downstream (when there is slack water between you and the current), and how to reach cast, micro-mend, mend on lakes where wind creates surface currents and all that. The basic mend is a fundamental building block for great angling. Perfecting that move will increase your hookup percentages dramatically.
Orvis 101 Introduction to Fly Fishing
Orvis 201 Streamside