Glucosomine and Chondroitin make great joint supplementation, as well as MSM.
Flax of course is great for the coat and provides Omega 3s.
I have done a lot of reading on DE, but have never used it. From what I have read, it doesn't do much for worms, but can be excellent for internal parasites. The key is to make sure the DE is dry and that you apply it to a dry dog and/or dry bedding. As soon as it gets moist, it is useless in the capacity of killing fleas/mites/ticks.

Jack