Some of the hitting the arm guard is related to the type of bow you shoot. Howard Hill said the string should bite your arm a bit, which is why he wore an arm guard. I always figured if it was okay with Howard, it is okay with me. I just go a bit longer with my serving. If you are shooting well, don't mess with your form just to save wear on your string.
The wear can also come from the string sliding across your chest if you shoot with a closed stance. I notice more wear on my strings in the fall and winter than I do in the summer when I'm just wearing a tee shirt. Add a bulky jacket, bibs, and a safety harness to the equation and your string will probably drag across your chest a bit. It never seems to affect my accuracy unless the string catches on a hood draw string. I don't have any of those on the coats I hunt in, but sometimes during practice I will. That's how I figured out where the string wear was coming from well below where the string could catch my arm guard. Traditional shooters cant the bow more, which adds to the chances of the string catching your chest.
I just keep the string waxed, and if it gets too fuzzy, I make a new one. Strings are about the least expensive thing we need for our type of shooting. If you make your own, they are even cheaper. If your form works for you, just keep an eye on the string, wax it often, and replace it regularly.