Whenever I shower more often, I don't use soap. Dirt and sweat go away with normal water, too. Soap is to get oil and fat away, and that's not needed so often. Same for hair. Shampooing once a week is fine; to get just the dust out of the hair, brushing or rinsing with water is sufficient. If you shampoo too often, either the hair gets dry and you can get dandruff, or your head skin tries to protect itself against the drying and produces more secretion => your hair gets oilier more quickly => you need to clean it more often => vicious circle.
I would actually agree with you Eva, but I have "hard" water here, so even just jumping in the shower to rinse off dries my skin out quite badly, which is why I had to switch to a soap that had the body butter in it.