As a former AMHA show manager this is my advice for what you need:
PATIENCE
SENSE OF HUMOR
Exhibitors will be stressed - more so if they aren't doing as well as they thought they would
They will forget to enter and want to enter at last minute, or forget that they entered a class.
BACKUP PRINTER and cartridges. Back up computer is great too.
Lots and lots and lots of copies of the show bill.
First aid kit - especially headache medicine - whatever's your favorite.
Coffee and doughnuts are very welcome in the mornings. Tea too if you can swing it.
Pen pens and more pens. They seem to grow legs.
I always let folks run a tab on classes. Never had anybody skip out.
Top quality announcer. Someone who knows how the classes run, has a good voice, and how to pronounce names (horses included). you also need a gate person who is patient and can follow the classes. This ISN'T a role for a child or someone who has no horse experience, although it seems pretty easy.
A headset kit is great. - connected to the announcer, gate person, and office.
RUNNERS - 2-3 kids who are willing to run to the barns, ring, etc. and generally run errands.
Drinks and snacks for the workers. An ice cold Pepsi can go a long way.
Good luck!!