NEW YORK—Aspiring stand-up comedians often think that making people laugh requires a foul mouth. Could be, but most of them won't earn a cent at it if they can't clean up their acts.

As Jeff Lawrence told a group of somber would-be comics one evening: "If you have a great set and you don't curse, nobody ever leaves going, 'That guy was great, too bad he didn't curse.' "

"Just...