Make a check list and make sure you get everything ready the night before and do all your grooming then. Like braiding, last minute trims or bathing. If you want you can cover them when they sleep if you want to make sure they don't get dirty.
Arrive early to the barn the day of the show and do any last minute touch ups to your horse. When you get the the show make sure you have at least one person there helping you because it will get very stressful and there will be A LOT to do. Make sure you have lots of water, hay, and sawdust because you might not be able to find any there when it is needed. A trick that is used a lot is when it is hot and humid out add a bottle of gateorade or powder to their water. This will supply then with electrolytes.
There is a longg list of stuff I could type but that is just mostly getting ready for the show. Hope that helped a little, and good luck at your show