I am in the process of waiting for approval from my insurance company for breast reduction. I think the key is to work closely with your doctor. The insurance company wanted to confirm that I have shoulder pain and back pain which I do. I also indicated headaches. They have a formula to determine pain levels apparently. They also ask about the duration of pain (e.g. 12 months, etc.) They want to make sure that you have tried to alleviate issues before going through surgery. So, for example, they might suggest that you go through a weight loss regimen if you have large breasts due to being overweight.
My doctor confirmed that physical therapy and support devices will not alleviate my symptoms. Also, the insurance company asked about my age. Some insurance companies do not cover the surgery for those under 18 years of age. It seems to be very important for insurance companies that you are not just requesting insurance coverage for aesthetic purposes, i.e. so that your breasts will look better or be symmetric or that your clothes will fit better. They want the surgery to be medically necessary.
There is a lot to go through for coverage. The insurance companies also want pictures. The key is to work not only with your plastic surgeon but other doctors who can attest to symptoms such as chronic pain, and inability to exercise, etc. Even a dermatologist can confirm a rash due to breast size. I await my response from the insurance company.