Another way to avoid tough meat in a soup or broth is to never allow the liquid to boil. That is, keep it at around 200 degrees. This is sufficient to cook the meat to a safe temperature, but not enough to toughen it. Ideally, the meat temperature should be no more than 170 degrees.
I use skins and carcass, or bones if not working with poultry, to flavor the broth. The meat is cooked seperately until done to my liking, then added to the soup after everything in it is cooked and tender. that way, the meat is perfect and not overcooked.
I also use the same technique of chilling the broth overnight in the refrigerator, and removing the hardened fat the next day. The liquid has gelled, telling me that it has absorbed the collagen, nutrients, and protiens from the bone and marrow, and will taste great when seasoned. Then I add the veggies and grains, cook until tender, serve in bowls, and add the diced and cooked meat to the bowls.
It's a bit more work, but the results are IMHO, very much worth it.
Seeeeeya; Goodweed of the North