Depends on the horse.
My horse for example, to prepare for a rated shows he needs to get shod a week or preferably two weeks prior with Aluminum shoes. (He can sometimes be sore after being shod, so taking two weeks before the show mean he'll more then likely be right as rain come show week).
If he is on any coat enhancers that have Paprika in them, you have to cut that out of the diet as Paprika tests positive as Capsaicin. If not then you are fine.
I like to ride or lunge my horse everyday before a show or put him on extended turnout (like full days worth) just so he's worked and relaxed.
I empty out my trunk of everything that I don't need, I double, triple, quadruple check everything because I hate having to buy something at the show. We load the trailer the night before we leave of everything except horses. You call the braider two weeks before the show to make sure they are available.
At the show my horse needs to be lunged in the morning, and if time permits washed, dried and then ridden before my class. My horse takes a lot of prep. I like handwalking him around the grounds so I am confident he has seen and heard everything. Wednesday you can usually school over fences in all the arenas and I like doing that or having the assistant trainer do it if I can't be there.
Gordon Wright once told a student to take up swimming because he would never be a good rider, that rider was George Morris.