Bathe them and groom them really well. Plait their mane and tail and don’t do a rush job. Make sure to train a day or two in advance, but not enough to tire your horse. Make sure you’re prepared and have all of your gear and your horses gear ready.
I make sure they are clipped at least a week in advance, they day before a show I wash them completely including the mane, conditioner in the tail, wash out and put detangler in the tail and braid it. Muck out extra well the evening before and have a clean rug on the horse. Put all the tack, my show clothes and grooming supplies in the lorry. I like to have everything ready the night before. Morning of the show I plait the mane, never the day before because they pretty much all roll when I'm gone 😅