That's why I always respond with "Terrible!" or "Awful!" or, a personal favorite, "Worst day of my life!" combined with a big smile. Really throws people off and is an easy segue into more meaningful conversation. But it's mostly just to disrupt the monotony of "fine, thanks, and you?" Oddly enough, if I really am having a bad day, then I'll answer with the normal "fine, thanks, how are you?"
