With the deer in the headlights look, I just say "Can I help you?" if it's a negative look, but usually I just ignore them. Sometimes, if I'm in that kind of mood, I'll put on the most glazed over look and stare right back at them, mouth hanging open. Most times it isn't negative, it's curiosity, so I don't mind. When they start pointing and snickering is when I get the snappy comebacks ready.