If your child seems to come out of the shower with his head only slightly wet, yet assures you that he really did wash his hair – tell him you want to smell his head.  When he looks at you funny order his butt back into the shower and demand that he re-wash his hair – and this time that he use the shampoo which is conveniently located barely an arms reach from where he will be standing.  Then because you’re a mom and not an idiot, suggest that after he washes his hair that he might as well re-wash his body; and that this time it would be a good idea if he used soap on that as well.