Here's what my dogs get as a prevention for arthritis or when they have arthritis (which at the moment none of them have despite being nearly 10 and nearly 13):
Salmon and/or coconut oil
Glucosamine/chondroitin/MSM supplement
Turmeric (excellent stuff, also for other inflammatory conditions, my saluki has degenerative disc disease and turmeric works beautifully)
McTimoney chiropractic (realigning the spine and other bones and with that helping them weight bare correctly and minimise incorrect pressure on their joints and keeping muscles tension free).
Other things that can work:
Hydrotherapy
Green lipped mussel extract
Devil's claw
TTouch
Massage (and similar therapies)
Acupuncture
Magnetic collar
And of course most important is keeping their weight low to keep additional strain off their joints.