I need some flirty comebacks....

If it's not your personality, I don't think it matters. They are flirting with you because they are interested, not because they are waiting for the punchline. Just make sure that you maintain eye contact, smile, and if you think of something smart and flirty say it. But don't think that you have to. Think about it like this you might just be coming across as mysterious, and that is more sexy than stupid wise cracks.

Oh and to the guy who moves your desk, say thank you to him like that chick in the movie Closer who was the stripper. The way she says it is hot, and sexy and very flirty; and all she says is thank you - unbelievable! I do that all the time, if it fits and I can't think of something to say.