And not spook off your main line and weight. Speed and depth of water determine length of leader. Try to keep as much distance from the fish as possible as they are a little smarter than most folks give them credit. Distance also allows them to feel safe and comfortable and not so pressured. They are much more likely to stay longer, giving you more shots.
Presentation is everything.
I find the kings the least aggressive so putting it in their face is best. Everyday, when site fishing is possible we find some fish that no matter how many times you present the fly to them or how many times you change the pattern of the fly, they simple are not interested. Hope this helps, if not e-mail me to discuss further. I'm always more than happy to help anyone who wants to learn how to fish.