Biggest issues that i've seen over the years as a fly fishing guide is that most people bring the rod way to far back or forward (10:00 - 2:00 or 11:00 -1:00 depending on what your casting), and people don't wait nearly long enough for the line and fly to "load" the rod on the back cast. The fly and line have to extend fully behind you before you bring it forward.
It doesn't take a ton of effort, most people try way to hard. The line is the weight that you are casting so you need a certain amount out to load the rod, usually about 20' - 25'. Those are some main points to think about.
Fly casting lessons are great too. An hour with someone standing behind you harping on you when you screw up helps a lot.
