Sometimes, we think of the best material only after the fact. It's sad that that happens, but that's life. What you CAN do, however, is train your wit to be speedy.
I get a lot of practice because of my three roommates. When we hang out (especially when we're drinking), banter flies all over the place: We question each others' sexual orientation, we laugh at mistakes that we've made in our love lives, and we insinuate that we performed certain actions with each others' mothers the previous night that are illegal in several states. "That's what she said" moments are absolutely killer after specific statements and questions.
You just have to get fast at coming up on the spot with those things, and it just flows naturally into everything else that you do. Practice will unlock that speed that you're looking for. Pretty juvenile, but that's my method for it.

Victory needs no explanation. Defeat allows none.