
I've used green lipped mussell, G&C and devil's claw with my oldest, as well as varous painkillers. The most effective for her has been the devil's claw, she has 4 capsules a day now (33kg dog with severe osteoarthritis) alongside trocoxil as her main painkiller, and tramadol during the break from trocoxil (6 months on, 2 months off).
She also has chiropractic treatment, as with supps it won't prevent arthritis or cure it but it can make a heck of a difference - much like me and my back, Soli compensates for her worst joints so the others get a bit overworked and out of whack, and sore as a result. The chiropractic sets them back, easing the pressure off them and the arthritic joints.
Hydrotherapy can help too.