You have to be careful with glucosamine supplements, lots of them (and not just "generics") don't have the amount they claim, at least not in a useful form. I've found with my own supplements, I can really tell the difference between brands.
Fletcher has been on supplements, and pain meds as needed, for arthritis in his shoulders for about three years. He is only 9 The Joint Health from Springtime has been good for him until recently. I've noticed him having trouble getting up, and the old man walk, like he is trying to limp on both back legs at the same time, and falling when he takes corners too fast, so I think he's now got it in his hips, too. I've ordered him some Dasuquin. It has a much higher amount of glucosamine, and is kind of the "gold standard" of supplements. It is more expensive.
Another thing I've found that helps me (and so I am assuming it would work for the dogs too) is MSM, and it is pretty cheap.