I have been giving my old Lady a glucosamine supplement for years for her arthritis. It really helps. It does take a few months to see a difference, though
The key is to make sure you give enough. Food, treats, even supplements like Missing Link and Nupro (according to my vet), don't contain enough to have any benefit. The dosage for dogs is 500 mg. for each 20 lbs. of dog once a day. My 8 pound Lady gets 300 mg a day per my vet.
This is a neat site to bookmark with dosage for OTC medications:
http://www.walkervalleyvet.com/otc-meds.htm
If you look at glucosamine products, you'll see that many of the less expesnive ones aren't really a bargain if you give the proper amount. The best ones are the brand names like Glycoflex and Cosequin.