Consumerist
Bankrate has a good example letter you can use if your health insurance company denies your claim for based on a specific medical procedure. It's in "Madlibs" style, so you'll need to replace all the items in red with your personal information. The argument is based on the premise that the insurance company must not been properly informed with the full information, and requires getting additional specialist letter(s) testifying to its necessity, as well as including articles about the procedure. There's no guarantee of its success, but it can at least serve as an initial salvo before escalating your issue elsewhere.
Form letter for denial of specific medical procedure [Bankrate]
