Bathtime stress!

I am writing this after a long absence to ask my fellow mums for advice about what they do when it comes to the dreaded hair wash?

S is completely phobic about having water on his hair and so we manage to wash it about once a month. He screams, panics, thrashes about throughout the whole process, and it's horrible for everyone. Does anyone have a failsafe method that they use?

Needless to say, it's nowhere near as easy as this image implies, but I couldn't help posting it!