I remember there was a no-poo (or something like that) craze going on 1 or 2 years ago. The idea was that hair, especially curly and wavy (i have loose curls), are very vulnerable to shampoo, and there was no need for it. So i stopped for a whole month, only washing my hair with cold water (hot water damages the hair).
Maybe my hair would have improved, but after 2 weeks, i suffered from SERIOUS dandruff. I was scratching my head all the time, and it felt like sand particles were irritating my scalp. My shoulders didn't look too great with lots of the white stuff falling out. Even so, my hair was VERY greasy. But once every week, my hair would look perfect. Which was odd.
I stuck with it for a whole month, and then i just couldn't take it anymore and scrubbed really thoroughly using a good shampoo and conditioner. It took a few washes, but it went back to normal soon after.