I have read that milled flax seed is very good for the skin, and hair. It must be milled, or otherwise dogs can't digest the seed. Oil works as well, but doesn't have the added benifit of aiding in disgestion. Two products that contain milled flax seed are Missing Link,
The Missing Link Canine Formula Super Food Supplement at PETCO, and a product made by Summerwinds called Healthy Coat.
Summerwinds - Suppliments. I don't know if either one of them work.