I also have a 9 yr old GSD with arthritis. She had TPLO surgery for an ACL tear a few years ago. Ever since then I've been giving her Glucosamine/Chondroitin/MSM. My vet stressed the MSM part. Here's the supplement that I use for my dog....it's a liver flavored chewable that I can just give with kibble:
http://www.drsfostersmith.com/produc...8&pcatid=14068
I've also had it recommended to give fish oil as a supplement to her kibble. I've actually just figured it out and will probably order it this week. After a LOT of research, the best one I've found is this (it has EPA, DHA & vitamin E):
http://www.seapet.com/seastarrstore/...products_id=84