Talk to your vet and rule out any pain that can be easily treated. See what is left as to what types of pain she is in that can't be treated. Some people choose to do hospice care for their animals. They somehow think that the animal has quality in its life simply because it can still wag its tail at its owners. I personally wouldn't prolong suffering, if I could treat the arthritis pain but the dog was still deteriorating in other ways I consider that prolonged suffering. You have to make the decision on your own though. Its a tough choice.