A very informal question would be "How's it hanging?" - very American and slang.
As far as answering "How are you?" when you're not feeling that good, you can say "So so" or "Not so good". However, remember that particularly in the UK, when someone says "Hi, how are you?" it is generally meant simply as a polite greeting, not a genuine enquiry into your state of health!! The usual polite reply is just "Fine, thanks". A good friend would probably be happy to hear how you really are!
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