Hi Bora
Remarkably, I've just stumbled across a video on YouTube showing an astronaut washing her hair aboard the International Space Station! Doesn't look too complicated, to my surprise.
There seem to be a remarkable number of videos on YouTube showing everyday life about the ISS, including crew members drinking water, eating oranges etc. It's quite a resource for life in space, to my surprise. Anyway, the hair-washing one is my first attempt on this forum at posting a link to a video. Here goes:
http://www.youtube.com/watch?v=h0dw-IzKxWU