My tip for white welsh A shining:
After Fairy Liquid, which has already been mentioned, I use sheath cleaner shampoo. Its great on filthy manes and tails. (just keep it away from eyes.)
I would do the following: 1/ Pre, pre show bath, bath. 2 or 3 days before. For deep level grease and grime removal, ingrained poo stains, manes and tails.
2/ Pre show bath. Day before, for last minute stain removal, use plenty of human conditioner in mane and tail. apply show shine for stain repellant, and mane tail conditioner after rinsing.
3/ apply hood, bandages, cooler or fleece rug, plait tail into 3 plaits, plait 3 together, put the leg from a pair of tights over the tail, (dont fold the tail, it will dry with a kink in it.) and a tail guard/wrap on top. (never leave tail bandage on overnight) Make sure bed is spotless and skip out last thing.
4/ Last minute titivation. clean bum, legs and any other bits which got shitty in the night! Re apply all of the above.
5/ If pony has a tendency to get the runs when excited, bandage legs for travelling, rather than boots, the hind boots will end up filled with shit and you will need to wash again!
Tips: Make sure that all horse clothing is spotlessly clean, including the headcollar, and make everything as pale a colour as possible. I have, in the past, uncovered Dpony on show morning, to discover her navy cooler has turned her sky blue!
Arrive at the yard ASAP, Dpony has a habit, only ever on show days of slipping her headcollar, pissing off across the neighbouring campsite and refusing to be caught... repeatedly. she is now lashed into her headcollar with extra straps! You also need to allow drying time for last minute washed bits.
Make a list of EVERYTHING that you could possibly need for pony, DD, you, the trailer and the rest of the family, for every eventuality. Sods Law rules at shows.
Remember to put stirrups, girth and bit back on tack after cleaning. remember water buckets/water/haynets and poo scoop.
Remember to pack joddy clips, gloves, raincoat and hairnet for DD.
If its hot, I freeze bottles of squash and keep in the water bucket for DCs.
Get to the show early enough to register for classes, unload and warm up pony. Do not get DD into her show clothes until the last minute, and have over layers for between classes.
On show day, not much will happen. You will spend ages standing round stressing, and trying not to breath on or touch the white pony, DD will do her stuff, then you will spend more time stressing, ad infinitum!
Dont take any notice of Posh Mum and her snide comments, she will just be trying to unnerve you. At the end of the day, your DD is a threat to little Clarissa's unbroken record! 
Dont turn into freaky/pushy/competetive/obsessive mother, you will only upset your child. who? me? never! 
And above all else, ENJOY YOURSELVES! 