Setting: The checkout at the video store. I’m second in line. The woman speaks to the female clerk.
WOMAN: Do you know where I can find a DVD copy of The African Queen? I can’t find it anywhere.
CLERK: There’s a Japanese release. It has Japanese packaging and an option for Japanese sub-titles, but it’s still the original movie. Let me just check about availability.
(beat as she searches on her computer)
Yes, here it is.
WOMAN: And it’s exactly the same as the original?
ME: Well, except for the car chase they’ve added at the end.
(Woman stares at me in disbelief for a moment before she laughs, while I feel immensely pleased at having managed for once to deliver a good line on time, instead of thinking of what I should have said six hours later).
Off the Wall 
I once read that delivering good lines on time is like fencing in that it requires practice (i.e. trying to think about punchlines often) and gets better with each time 🙂
Good one, Bruce!