+ive:
not too bad
feeling good
fine
good
neutral:
Same old
Same as usual
Can't complain
-ive:
Not so good
I've been better
I've seen better days
ALL of the +ive replies can be followed by "thanks", "how about you?", "yourself" ...etc.
for e.g. Feeling good, thanks, how about you?
neutral: don't say "thanks" after but you can ask the same back.
e.g. Can't complain, how you doing?
-ive: Again, don't say "thanks"
e.g. I've seen better days, and you?
Of course there are many more answers you can give...PLEASE don't feel you HAVE to say "fine, not fine" all the time! End up sounding like a robot! Just say what you feel.