Use shampoo and conditioners designed for damaged hair. Curly hair is dryer than regular hair, and because of this it's more prone to damage. It's a good idea to baby your hair slightly during the shampooing process if you have curly hair. Purchase shampoos made for damaged hair, which include milder ingredients that are less taxing on hair.
Make sure to buy salon products and not drugstore products; drugstore shampoos and conditioners are often diluted, making them less effective.
If you can't find a shampoo for dry or damaged hair, some stores sell shampoos for curly hair specifically. The formula in these shampoos are similar to the formula in most products sold for damaged hair