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Dandruff can be a symptom of dry skin, seborrhea or allergies. If this is due to dry skin adding omega fatty acids to the dog's daily diet will help in a matter of two to three weeks. http://www.petplace.com/drug-library/fatty-acids-omega-derm-caps/page1.aspx You can find many dog products that contain this in your pet store.
Some dogs breeds are prone to skin problems such as seborrhea, if this is the case a vet visit would be in order. http://www.petplace.com/dogs/seborrhea-in-dogs/page1.aspx
Air borne allergies present another problem that can cause dry flaky skin and itching. This should also be determined by your vet.
There is also a mite that looks like dandruff so if the dandruff is moving then it is a mite infestation. http://www.petplace.com/dogs/cheyletiellosis-in-dogs-walking-dandruff-mite/page1.aspx
I would try the omega fatty acids first. If that does not work then get the dog checked for other possible problems.