When I was beginning in the martial arts, the stretching methods were not yoga-based, so instead of stretching muscle we were stretching joints to the point of damage. Now, when I twist to position myself on a ladder from a roof, or when I enter a crawlspace where I need to squat and duck-walk, my knees pop out readily. The pain is excruciating, and I have to fall back and straighten my leg immediately in order to pop my knee back in.
But for maintaining a well "lubricated" and healthy joint system, diet is also an important factor.
Essential Fatty Acids (omega-3 and omega-6, for example) increase production of anti-inflammatory prostaglandins;
Bromelain is an enzyme that helps stimulate prostaglandin production and reduces inflammation;
Chondroiton sulfate strengthens joints, ligaments, and tendons;
Glucosamine sulfate for the formation and regeneration of tendons, ligaments, and cartilage;
S-Adenosylmethionine (SAMe) helps maintain cartilage and reduce pain and inflammation;
Most important vitamins and minerals:
E, B-complex, C, Calcium, Magnesium, and Copper.
From: Prescription for Nutritional Healing 3rd Edition: Phyllis A. Balch, CNC & James F. Balch, MD; 2000