Stay healthy & light. Many uncontrollable factors can contribute to developing osteoarthritis such as genetics, family history, trauma and injury. However many things can be controlled and prevented. The best things to do are to stay in excellent shape, stay thin, and have a good diet. Mix weight bearing and non-weightbearing (swimming, biking) activities rather than continuously pounding on the joints on hard pavement.
Preventing Arthritis. We know that joint trauma, aging, obesity are factors that promote the osteoarthritic form of arthritis, sometimes as early as the thirties. Protecting your joints by choosing healthier exercises patterns when younger, continuing to exercise with your joints mildly (proven to help), keeping your weight down to normal, all help.
Nothing Yet. There is nothing we know of that will prevent the onset of arthritis. There is a lot of research being done but nothing that has great promise as of yet.