This is probably one of the really good tips of hair care. Basic chemistry states that oil and water do not mix together. So, this principle applies when wahing off hair as well. If you wet the oiled hair or even non-oiled hair (it has natural oils), the shampoo doesn't react well with it and the hair more or less remains dirty.
Since commercial shampoos are filled with surfactants, they are strong enough to clean the hair but the SLS-free shampoos generally end up not cleaning the hair properly. So, how to use the shampoo on dry hair? Take a dollop or two of your shampoo in a small container and add just a fist ful of water to it. Dilute it nicely and shampoo your hair just as you would.
Always try to shampoo twice if your hair does get exposed to lot of sun and pollution. First wash only cleans the scalp partly and the second it to make the scalp squeaky clean. And, by squeaky clean, I don't really mean that it should squeak but a clean scalp and hair is very important for healthy hair. If you tend to stay inside more, one wash is generally enough and you can do it twice in once a week.
So, don't forget this tip next time you hop into the shower. It generally happens with me that I remember this when I pour water over my head so don't make that mistake ;)