I’m bored of saying “fine” every time I’m asked this. I’d like something positive and uplifting, rather than negative or snarky.
So far the best I’ve been able to come up with is “If I were any better I’d be twins.”
The receptionist at our office often replies with, “I’m so happy I have to sit on my hands to keep from clapping.” Cracks me up every time.
Its not a question, its a greeting.
Fine, how are you? Is the correct response.
I like “Quite how, thank you.”
“Hangin’ loose and full of juice”
Awesome!
I know a woman who responds to “How are you?” with “Blessed”. She’s a pretty cheery lady.
I’m in the UK, and I often try to give a basic response embellished with a bit of information as an opening to conversation if the other person wants. Something like:
Hi! How are you?
I’m good! I just [interesting fact]
But then I visited America, and discovered that what everyone said is true. “How are you?” is more like “How do you do”, and replying to “how are you” literally is nearly as weird as to “how do you do”? (“I don’t know, how do you do what?” :)) If I tried the above, it’d be more like:
How are you?
I’m good! I–
I’m fine. What can I do for you today?
So in short, I’d think about giving up on snappy and settling on maybe “great” 
Gah, I’m starting to positively *hate * that. In my neck of the woods, it’s generally said by sanctimonious twits. “Have a blessed day,” pronounced in two syllables: “bless-ed.”
Sorry, personal problem, I know.
I like to reply, “I don’t know. I’ve never tried me.”
That usually gets me a general WTF look.
Works REALLY well with upper management.
I hate this! Hate it.
Since I moved to Atlanta, I hear this on a regular basis. Ugh.
My smart-ass answer to “How are you?” or “How are you doing?”:
“I’m rockin’ pretty hard. I’d give myself about a seven and a half. Maybe an eight.”
If they’re game, they’ll often follow up with: “Seven and a half? What’s that out of?” (It’s surprising how consistently I get this.)
To which I’ll usually respond: “Three! WOOOO!!!”
“Your attempt at social interaction is hereby acknowledged.”
Some of the things I say:
[ul]
[li]Just peachy keeny ducky![/li][li]Fantastic! (in the Dr. Who voice, of course)[/li][li]Outstanding![/li][li]Fabulicious![/li][li]Hunky dory, morning glory[/li][/ul]
Yeah, I roll like that.
roflmao. I could totally hear my SO saying that. I don’t have the dry wit to carry it off. I’m too bubbly.
Maybe one day you’ll be lucky enough to find out. 
I know someone who always says, “I’m better than I was, but not nearly as good as I’m going to be.” He’s a deliberate positive thinker. And I see him seldom enough that I forget and walk right back into it.
I’m doing rather well, thank you.
This made me laugh so hard.
“Spectacular, by all reports!”