PM'd advice from an old timer....
Real longlining is an art....rather than science. Key is to keep your eye on the load and fly the load not the helicopter per se.
An analogy is balancing a vertical stick on your outstretched palm...as the stick falls right....you have to move right to get back under it...which ever way the load goes...that is the direction the helicopter needs to go to stop the swing. Forget trimmed movements in forward flight....stick to cross controlled lateral movements to get over the load as it swings. If in forward flight....and a swing shows up...slow down a touch....or apply a bit of collective...up or down at the very bottom of the swing....not at the ends.
Lean way over....door off is best unless you have a bubble window....and set the seat higher and more forward than you would normally fly. (if possible).
You might even have to slide to the outside a bit and rotate your body to the rear so you can get a view that is square to the side of the aircraft
Find a reference point(s) on the aircraft which is center datum for the load and keep the load there by moving to the load with the aircraft. Once you have your target on the ground or track to be flown (assuming a firebucket)....fly the load to the target or along that desired track.
170' or 407Driver are probably better suited to help you out.....I qualify as a serial killer when it comes to longline flying.
If you are trying to use a mirror.....and assuming you are talking a longline....somewhere between 150-250 long....Vertical Reference is the best method and forget the mirror. Up to about 70' of line....a very good mirror setup might work. The real trick is to get your head out so you can see the load the entire time you are flying. Enroute I used to get inside but on hover, takeoff and approach....get outside.
The best way to practice is picking up a nice dense thus "stable" load....hover a few feet above a finite point....something that is easy to see and aim at.
Practice picking the load up to a ten foot hover....keep the load over the point....when you get bored or frustrated with that....pick two points a bit apart...and start landing the load on each point in rotation...back and forth....really focus on flying the load to the target in one smooth move rather than hover...move...hover...move...hover..move...the less time you hover the less the load will swing. Enough practice and the movements to fly the load will become second nature and thus just like hovering....much easier to do when you quit thinking about moving the controls.
If it becomes a dog's breakfast....go for tea and relax. Don't fight it.
I have worked with 300 foot lines a bit....they are in some ways easier than the shorter lines...as things happen much slower...but the delay between a movement at the top and a reaction at the bottom is significant.
Another hint....if you are using several lines joined together to make the longline....take some U channel or angle iron or metal pipe....and a bunch of duct tape and brace the joins with the bars/pipes and duct tape to make the line act as a single line vice two or three lines that move in the middle rather than only at the ends.