Maybe make a game out of it? My mum used to make me tip my head way back and keep counting the tiles on the ceiling/just see if I could stay fixed on one point in the ceiling while she distracted me with kisses, tickling, and washing my hair. Also, since we were lucky enough to have a tub, she'd let me make "foam art sculptures" with my hair while sitting in my bath, thus beginning my journey of epic, wild hair at a very young age.