Definitely make a list and pack, I do this wired thing where I get paranoid that I haven’t packed everything, so if that happens, just BREATHE!! And check your list again if you have to, but make sure that you check off the items when you have packed them! Wash your tack and eat and breathe and sleep;) hope this helps! P.S. Also go over your jump course if you have it!
Make a long list, pack, and do breathing before you ride!
Before a show I always try to ensure I am as prepared as possible to keep my show nerves to a minimum. I will bathe my horse, clean tack and equipment and pack everything into my horse box. I also sit and visualise my test. Sometimes with showjumping it can be hard as you don;t have the course but you can still make sure you visualise having a smooth warm up and ride. I like to get lots of sleep and make sure I have a healthy meal and have packed some snacks and my ring bag. I also like to watch youtube videos of riders I admire to help get me in 'the zone'.
bathe and pack/clean tack
One thing that I know that really helps my horse is just having a little lesson going through some of the movements that we'll get in the test (I do dressage) but I try to make it easy for him like sometimes using a stronger bits which help him keep his head down and in turn keeps his frame good which improves the movement.
What this does is, it allows the horse to feel that what he's doing is easy and it let him know that he can do it.
So maybe just doing something that puts him in a comfortable place with whatever the horse has to do for the show, weather it's jumping or riding a dressage test. Just keep in mind to not over work your horse because it might leave him/ her stiff.
Best of luck, hope this helps.
Make sure to put everything in your trailer and give you me horse a bath
Before a show, make a checklist and make sure you have everything on the list!