Regularly bathing your dog with a good quality shampoo is key to the overall health of their skin and coat.
How often depends on your dog's skin, for a dog with dry skin, once a month is optimal. Using a harsh shampoo, one with lots of synthetic ingredients, or worse, human shampoo, strips your dog's skin of its natural oils, causing dry skin and lots of itching.
The best type of shampoo to use on a dog with dry skin is a natural one containing Colloidal Oatmeal. Even better is to follow up with an excellent moisturizing rinse. Just as we use moisturizer on our skin to combat dry skin, a moisturizing rinse will do the same for a dog's skin.
I recommend using Nature's Specialties Colloidal Oatmeal shampoo and Colloidal Oatmeal Creme Rinse.
To make your shampoo last longer, and avoid over shampooing, simply dilute in warm water using a shampoo mixing bottle. This also makes bathing more manageable and allows even distribution of the shampoo allowing it to lather correctly.
You can also follow up with Wham Anti-Itch Spray for those really itchy areas or for in-between baths.