I looked into it once for OEGBs (I was curious)
With OEGBs, the roosters show pretty naturally, and I'm going to guess this is the case for most bantams. The provisio was that you want the judge to be able to reach him without chasing him around the pen for five minutes, so you have to spend some time getting him used to being caught and handled.
The hens are more timid, and some things were said about training them to tense and strut when someone reached in--poking them with a pencil (eraser end) was the recommended method.
That is, of course, for OEGBs.
Thankfully, you do not have to dub your birds for show. I would have to dub mine.