I had to sign a waiver once because I asked that they cook a burger as rare as legally possible.
I also use that phrase regularly, to good effect. A few others I've used:
Haven't ever had a bad burger or steak yet using the above. OTOH, I've been cursed with some horrendously overcooked items over the years when I've forgotten to specify. "Rare, please." "Are you sure, sir?" "Yes, but if you can't serve it rare, I'll settle for raw and a Zippo."