Have you tried the adequan (not sure if I spelled it right) injections? They work wonders. If I remember right you give a dose once a week or once every other week for the first 4-5 times then after that it is given as needed, ussually once a month or so depending on the severity. It is semi-costly it costs my 78 lb dog $40.00 a shot, but well worth it!
Also massage therapy is wonderful too.