Dandruff is caused because of either when the scalp gets too greasy much or it gets too dry. Scalp form new cells continuously and with dandruff this regeneration increases and the dead cells shed off as dandruff. At times it is caused by dermatitis or eczema or allergy to hair products.
Try using medicated shampoos containing ketoconazole , salicylic acid, selenium sulfide sulphur etc. Shampooing on regular basis helps remove the dead cells and prevents building up dandruff.