In all seriousness, as far as I know, it's selective breeding down. You take the smallest of a breed and breed those to each other, and then take the smallest of their offspring and breed those, and so on down the line. Do this for several generations, and voila! Bantams.
I'm certainly not an expert, but from what I've heard that's how it goes.
Bump for the afternoon crowd. I think I may have hatched a bantam out of my LF eggs if the above statements are accurate. I bow to the superior collective knowledge of the borg...er...I mean flock.