Many times a dog's coat will become kind of dull and dry if they are not getting enough essential fatty acids. Omega 3-6-9 are destroyed in the high heat processing and storing of most commercial dog foods.
The easiest way to keep a dog's coat silky is to add a good coat supplement to your dog's diet. These are usually made from fish oil (probably the most popular), primrose, borage, and flax oils. Look for 'coat supplements' and look for cold-pressed oil as they retain the most EFAs.
Personally, I have also used raw eggs with great success in helping the coat. However, one of my dogs developed allergy symptoms, so that was one of the first ingredients I cut out.