Man: "Haven't we met before?"
Woman: "Yes, I'm the receptionist at the VD Clinic."

Man: "So, wanna go back to my place?"
Woman: "Well, I don't know.  Will two people fit under a rock?"

Man: "I'd really like to get into your pants."
Woman: "No thanks.  There's already one asshole in there."

The most memorable rebuttal to a turn down (used by the guy who
used to live across the hall from me in residence) when he asked a
girl to dance and she refused:
Man: "Want to Dance?"
Woman: "No, thank you."
Man: "Don't thank me, thank God somebody asked you."

Man: "I'd like to call you. What's your number?"
Woman: "It's in the phone book."
Man: "But I don't know your name."
Woman: "That's in the phone book too."

Man: "So what do you do for a living?"
Woman: "Female impersonator."

And here's one including the correct snappy return
Man: "How do you like your eggs in the morning?"
Woman: "Unfertilized, screw off!"

A girlfriend of mine once had a graying man in his 60's approach  her in a
club while she was in college with the line "Where have you been all my
life?" She took one glance at him and said, "For the first half of it, I
probably wasn't born yet."

A friend of mine came up with a very quick response over vacation. We
were walking down the street and I glanced at a girl who had just
walked by.  She turned around and said to me, "What are you looking
at?"  My friend, walking next to me came to the rescue, "He thought you
were good looking, but he was mistaken."

While at college, a few friends were discussing how their "passes" had
been rejected by the intended female receiver.  One of the ladies
explained how she handled it once... When the guy, obviously getting
irritated, blurted out something like, "Hey, come on, we're both here at
this bar for the same reason!"  She responded, "Yeah! Let's pick up some
chicks!" He immediately blanched, and decided that maybe  he would look
someplace else.