I don't know if it will help, but I tell DS that he's taking polyjuice potion. Then he gets to turn into his favourite death eater (usually Bellatrix) - sometimes Harry Potter, Hermione or Ron, but usually a death eater. The potion wears off when my patience wears out. (Plus that stuff is supposed to taste foul if you're becoming a death eater.)

DS also takes the spoon as soon as it is in his mouth, so he tips the medicine down his throat.