My favourite variation is, "Can I ask you a personal question?"

Which, I now realise, isn't even that helpful, given that a "personal question" could be anything from "what's your first name" over "Do you realise that the spot on your forehead which you've half-covered with make-up is spectacularly ugly and visible from Outer Space?" to "What's your weight and bra size, and do you shave your pubic hair?".