Well, a lot depends on the type of longlining - you have operational longlining, which just about any competent pilot can do with a little training and planning, there's precision longlining, which pretty much means what it says, and there's production longlining, which is like poetry in motion when done properly and takes at least a week to get into before you can be productive.
For the basics, you don't even need to have your head out of the door too much - set your altimeter to 0, when you come up to the length of the line expect it to get taut, when it does, keep going up until you cleared the height of the obstacles you checked beforehand and away you go. Just make sure you have a good margin of power, 'cos you're doing a towering takeoff. At this stage you might want to stick your head out to check the load is off the ground and maybe guide it around a couple of trees (or not nearly hit a hovering eagle up the *rse as I nearly once did
), but it's not essential.
Coming back to the ground is the interesting bit, and you should always have the load moving, even if it is only up or down. Again, you know the height of the obstacles, and the landing pad, so you're using your altimeter again, and you should aim to get the load going slightly in front of you so the effect is that it's pulling you toward the spot, albeit slowly.
You can line yourself up over the landing spot in general by using a marker in front and one to the side - only in the later stages do you need your head out for more precise positioning, and even then just a quick look.
If you fly crabbed off, the sideways presentation of the fuselage will help slow you down, as will use of the collective only, as used in mountains. Particularly important is to coordinate the controls together just before you plonk the load on the ground, especially in a 350/355. If you use collective, then pedals, you will get a swing immediately. Keep that nose straight!
Longlining isn't rocket science, but, like piano playing, the essence is in the practice, not so much the method, and it is something you need to keep current on.
The most satisfying bit to me is getting the empty hook right next to the rigger's hand!
Phil