I generally wash my soft feathered poultry the day before a show - preferably in the morning. I use a show shampoo for my horses. With the white birds I'll set up three tubs. One for the shampooing, one for a blue rinse (I use bluO, but I'm not sure if it is available in the states), and the final bucket with vinegar for the final rinse off.
(Be careful with a blue rinse... too much will dye your birds blue!)
I get a nail brush or old toothbrush for gently scrubbing their feet, beaks, wattles and combs.
After towel drying (rubbing only in direction of the feathers.... being very gentle) I put the birds into crates with clean shavings and set up little electric heaters to blow across half the pen. They will generally preen themselves in front on the warm air and dry by the evening. I'll turn the heater off as soon as they are dry underneath. You have to be careful that things don't get too hot for them.
With my hardfeather varieties (game birds) I'll only wash them once at the start of the show season. They are then kept in clean pens so that I don't have to wash them again. They do better if you allow the natural oils to return to the feathers because it will give them a glossy sheen. Too much washing will take away that lovely shine you are looking for.
Morning of the show I'll do final preparations before I pen the birds. This includes a final foot scrub, face wash, trimming of any little prickly feathers along base of the comb and a polish off with vitamin E cream on their red bits. I'll also put vegetable oil on their legs and polish off with a dry cloth.