When showing you are not normally allowed food or water in the pens until after judging, and restrict access to food from about 10pm the night before meaning that the birds are not going to have a massive crop affecting their shape, and are less likely to poo in the cage getting it and themselves dirty.
I have about 6 birds entered in next weeks Fife Show, i do nothing really special when showing them, i clean their legs getting rid of any muck, and cover them lightly in Vaseline, i also trim toenails and tidy up tail feathers ever so slightly. I would not worry to much about the bald rear, i put one in with a (scabby back only noticed once she was in the cage) and she didn't come last. its the taking part and enjoyment that matters, so long as she is not ailing from it.
I allways start my birds on a Lice/mite paracite before and after showing to make sure i do not take anything to, or back from the show.
make sure you take your own bedding for the cages(just enough to cover the bottom) as you never know what stewards provide, but make sure you clear it up afterwards and dispose of, and don't make to much mess with it.
If the tent get to hot you can tell the steward manning the tent and they may be able to remove a side, and don't be afraid to tell someone if you spot a problem such as an ill bird etc. a few years ago at the Ryedale show a bird in the cage dropped dead next to mine, and no body moved it, not even its owners. so after a serious complaint the birds was removed and it's owners ridiculed as the had not provided it with adequate water.
hopefully this help but feel free to pm with any other questions.
Good luck and most of all HAVE FUN.