I don't show "big time" and just go to the local ones, but I still try to let me and my horses look nice.
I don't have a large size clippers, so a week before the show I will use a scissors to trim the "feathers" on their pastures. I do it a week out so that my scissor marks can have time to blend in.
In general, I keep their manes and tails braided with very little brushing (except show day) to minimize hairs getting ripped.
A couple days before, I make sure all my tack is in good repair and any tack that needs to be washed or cleaned is taken care of.
The day before the show, I use my mini clippers to trim their muzzle hair and around the eyes, and clean up the long hairs around the jaw. I just "tidy" their ear hairs. I don't show big enough to trim their ears fully, but just take care of longer hairs that are sticking out to make it look neater. If I have missed any, I can easily touch-up the day of the show.
The day before a show, I may or may not give my horses a full bath. Depends on the weather and if they need it.
I don't have to pack my trailer because that is where I already keep everything, but when I used to live at home with my parents, I would pack the trailer the day before or else set everything in a pile in the barn so it was be ready to go in the morning.
The morning of the show I get up extra early to wash 7 white socks.
And use white brightening rinse. Once their legs are totally dry, I will apply hoof polish.
They get brushed from nose to tail, and all their hair is brushed out. I will usually take a damp cloth and just wipe down their entire body to wipe off any surface dirt. When their hair is dried completely, I'll spray with show sheen. I wait to do a "final brush" with my soft clean brush until I get to the show.
I'll take a damp cloth and wipe off any nose boogers and eye crusts, and will do so again at the show if need be.
I don't change into my clean show clothes until my horse is ready and my class is coming up. That also includes CLEAN boots.