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I'm fine 

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I'm alright.

I'm OK.

I'm good.

Pretty Good.
I've been better.

Can't complain.

Hanging in there.

Not too shabby.

Fair to middling.

Not bad.

Not too bad.

Not the best.

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Bye-Bye

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Say Bye or Good Bye instead of Bye-Bye. Bye-Bye is childish.

See you!

See you laster.

See you tomorrow.

See you on firiday.

See you soon.

See you in a minuite.

Tara!

Tara for now!

Bye for now. (I'll see you again soon.)

Cheerio! (British)

Later! (Very informal)

In a bit! (Very informal)

Right! I'm off. 

Have a good evening.

Have a good week.

Have a good one!

Have fun!

Humorous ------------------

See you later, Alligator! ( later -> alligator )

In a while, crocodile!