Longlining -
Just 2 cents worth from here and there after about 1000 hours of it.
In no particular order -
You have to learn to hover by looking straight down or in some cases behind you, it's just different. The helicopter is doing the hovering, your just the pilot.
Forget the helicopter and just fly the load as if it is the helicopter - sounds oversimplified but it works.
If the load swings place the helicopter over the top of the load at the end of the swing - it will stop. If it swings again - rinse, repeat,rinse repeat............................ Ask a crane operator to show you or try it with a piece of string and a weight.
Depth perception / judging height. On a sunny day when the shadow of the load meets the load it is on the ground! When the shadow is 20' away it is at about 20' above the ground give or take, it's just a guide. After a while you won't need this.
You really only need your head out the door when picking and placing the load. As you increase power let the aircraft move to the apex point over the load, it will lift up clean and straight. Take a very careful note of how much power it took to pick it up, and don't forget it!
Never use flat webbing slings - they can flutter in the airflow and will shake the fillings out of your teeth.
Be very careful with loads that have aerodynamic capabilities regarding their shape and weight. Big flat stuff wants to play formation flying with you!
Lots of guys in Europe, especially Switzerland longline by mirror alone. It just takes a little more practice and avoids RSI of the neck.
Learn how to be smooth on the cyclic and not "stir the pot" it absorbs power.
Never use a line that will stretch - many before you have tried. Most of them aren't around to tell you anymore. If you are using a line constantly under hard conditions it will wear. I prefer wire crane cable (as opposed to wire rope) or chain.
Have enough weight in the line or on the end so that when flying with no load it is stable and remains well clear of the aircraft and in particular the tail rotor!!!!!!!!!!!!!!!
If you wish to source equipment, hunt down somebody close by geographically who can help. It's a mountain thing so North America, Central Europe, PNG, NZ......... etc are good places to start.
Very challenging, yet rewarding flying, especially when you get to the stage of being able to place the hook in the hand of a seismic drillhand who is 100' down in the trees and not even looking up for it while you have a 150' line on, in one fluid motion. Do it for 9 hours a day and anything is possible.
Appreciate I have left out the blatantly obvious!