There are quite a few different causes of itchiness in dogs and they require different treatments. It is very difficult to give advice without examining the dog. It is always best to see a vet where possible.
General advice would include the following:
1. Ensure very good flea control, even if you do not see any fleas. frontline is a very good non-prescription treatment.
2. Use a medicated shampoo to help control bacteria and yeasts living on the skin. Coatex Medicated Shampoo is a good example.
3. Provide a hypoallergenic diet. Sometimes this alone will make a dramatic difference, but for some dogs it makes no difference at all. It all depends on the indvidual causes for the itchiness. Purina Veterinary Diet HA is a good exaple.
4. Provide an oil supplement to the diet which can help to condition the skin. Efavet capsules are very good ones, though there are cheaper alterntives.