Damn, that's a hard ass question.
Let's see... I hope you have a reeeeal active imagination, that helps.
Not everybody can spit any punchline and get away with it... sometimes it's not the line itself, but how you say it that counts.
If you get one on the fly, just write it down, you can always use it later.
You may want to audition them in front of close friends or music coleagues, I'm real critical of my own, so unless I think it's sharp enough, I won't use it.
Usually they just come to me, or bit and pieces come to me as I write. But if I try to develop some, I just think of Impossible situations, insanely hypothetical, what-if.
You can use a metaphor or a similie as the basis for your whole line.
It's like thinking in Dozens mode, but you can expand it further, you just gotta tax your wittiness to the extreme. For me that's hard to do in a freestyle.
Plus you can use all that useless garbage they taught you in Highschool as fuel for more complex punchlines.
Bottom line..... if
you think it's corny, it probably is, you're your greatest critic........................................ or should be
Hope this helps,
do you want examples?
What's multi's?