There must be an eye in each end of a sling, preferably reinforced with steel, to protect the rope, and the shackle that goes inside the eye must be the right size for the hook, otherwise it may come out by itself, or not come out when it’s supposed to (as a guide, the shackle pin should be the same thickness as the rope – DON’T use a bolt instead; it will bend). Using a swivel will stop the load from spinning and unravelling the line. That is, you put the swivel in the hook, then attach the load to the swivel (which, by the way, has a thrust bearing in it). When using a screw-pin shackle (clevis):

with a rope of whatever description, don’t have the shackle downwards (as shown above), because the rope will slide from side to side and have a tendency to open the legs (if it rolls fore and aft, it will also undo the pin). Rather, place the screw-pin in the helicopter hook, padding the empty space either side with washers or similar, and use it upside down.
Chokers, technically, are lanyards that hold loads with a slipping noose which tightens under tension – the harder the pull, the tighter the grip. Use chokers at the end of a line rather than by themselves so that, if they break, they are kept away from the blades by the line (this also helps prevent collective bounce—see below). Be aware that a choker has 25% less safe working load because of the bend in the rope at the noose. The sharper the bend is, the less the SWL will be.
Long lines (see below) have an electrically operated hook at the end, which really makes them an extended hook, so there will be an electrical cable for the release mechanism, which should be carefully looked after; that is, the line and cable must be detached properly by ground crew, otherwise you will strip the cables, which won’t impress the engineers. If the insulation comes off, you may also get a short circuit. Lines made of Spectron or Kevlar are strong, but light, so will trail after you more than a steel line, hence the maximum external load speed. These lines also get longer when new, so, if you can’t pre-stretch them, allow a good length of extra electrical cable.