You KNOW it.

For some reason, I'm incapable of responding appropriately to the sort of light, breezy greetings that pass between people who pass each other in the halls, or on the street. You know, that sort of,
Person 1: "hey, what's up?"
Person 2: "Nothing much, yourself?"
Or,
Person 1: "How are you doing?"
Person 2: "Doing good, doing well!"

It seems so simple, but I can't seem to process in time to give back a sensible rejoinder, without an unnatural lag-time that makes it seem forced. Most of the time, I'd just guess - leading to uneven results:
Person 1: "How's it going?"
Me: "Nothing much, yourself?"
Or,
Person 1: "What's up?"
Me: "Outstanding!"
Or,
Person 1: "How have you been?"
Me: "Nothing!"

Over time, I fell into the habit of responding to all comers with something that may not necessarily fit, but it wouldn't be clearly wrong...because it wouldn't be the obvious response for any of the other common greetings, either. I've gone through several of these "designated reply phrase" standbys, but the one that's stuck with me is: "You KNOW it." It works pretty well for just about anything:
Person 1: "Hey hey! How are ya?"
Me: "You KNOW it."
Or,
Person 1: "What's up?"
Me: "You KNOW it."
Or,
Person 1: "Where have you been?"
Me: "You KNOW it."

I think that ends up being a more indicative answer than any of those cliché responses anyhow. It's indicative of me, of what's up with me, of how I'm doing, of where I've been.

That's just how I do it.