Glucosamine is absolutely fantastic for osteo arthiritis in both dogs and humans. I have dodgy knees and when they play up, I just go on it for a couple of months and I'm right. My dear departed Amstaff Freya also developed arthritis in one knee at around age 10 (did the cruciate ligament 5 years earlier) and she was put on the 4 injections a year. Wasn't expensive either, only $25 every few months. Can't recommend it highly enough.
Basically what it does is provide temporary lubrication around the affected joint, especially if cartilage has worn down. Also has anti-inflammatory properties. It's basically an amino acid I think, therefore, you don't risk the side affects of other anti inflammatories which can cause gut ulcers and the like.