I often sing silly songs to my daughter. One of her favourites is the "Shampoo" song. I think the silly song makes shampooing an eagerly anticipated event, even if she does sometimes end up with shampoo-y water in her eyes when it gets rinsed out.
Shampoo, shampoo
It's much better than real poo
Shampoo, shampoo
Let's put some in your hair