If you have hard water, using soap will leave your hair with a manky sort of gunky tangly film. I have soft water, however, and have from time to time used a couple drops of Dr B's, and it turns out great.
You can also clean your hair with conditioner. Nature's Gate Organics is the only line of "natural" conditioners I've heard of that works well -- you want a thin, runny type similar to Suave or vo5, not a rich thick one. Wet your hair, squeeze out the water and glop on handfuls of conditioner till your hair is all saturated and slimy with it. Leave it in from 10 - 20 minutes. Rinse, rinse, rinse. I set a timer for 5 minutes and rinse the whole time. Uses a lot of water but I only wash once a week anyway.