Shampooing too often is one of the most common causes of dry hair. Many people believe that squeaky-clean hair is healthy hair, so they wash it one or more times every day. But shampoos often contain harsh cleaning agents that can strip away your hair's natural oils, which help hold in moisture. On the other hand, a gentle shampoo will stimulate the oil glands, so you probably shouldn't go longer than three days without a good lather.
The ideal shampoos for dry hair have a pH of between 4.5 and 6.7, but here's a good rule of thumb: Don't use any hair cleanser that you wouldn't put on your face. Some people recommend baby shampoos, but their pH is usually far too high; such alkaline shampoos dry out the hair. Acidic shampoos are better for your hair.