I've been a part of organizing a few shows and then was the Show Secretary for another. Some tips:
Ribbons: Hodges Badges http://www.hodgesbadge.com/
Excellent product, excellent customer service. Do you also want prizes for high-point or anything?
Judge(s): Arrange EARLY (as suggested - like months and months early) and create a signed contract. You may need to pay for their transportation and hotel. Also, you should have a designated person to assist the judge during the show. Someone who's friendly and personable, but also who knows when to shut up and let the judge do their thing! They can take notes for the judge during classes (if requested), be a communication link between the judge and show committee (so the judge knows when to do what and where), and bring them lunch.
Parking: You'll need lots of car & trailer parking. Make signs/use cones for no-parking areas.
BBQ/Food: Don't do it! Or just hire it out (even a coffee booth). You won't make much (if any) profit and it's a ton of work. And you need to have someone manning the food booth all day. Sub-contract or you could have small things available for purchase at the registration office (bottled water/candy). But again, you probably won't make any profit. Or, contact your local 4-H, they may want to run a food booth as a fund raiser (they could get the food donated so they actually would
make a profit).
Restrooms: If the people are eating food, they're going to be poopin!
Do you have the facilities to handle all the extra restroom traffic?
What if it Rains: I won't know what type of weather you have, or what season you're planning this show for, if it could possibly rain, it will. Do you have a large area where exhibitors and horses can stand undercover? We used our local fairgrounds onetime (like Tim mentioned) and it was VERY convenient and had all amenities that we needed...but it was quite pricey and cut into our profits a lot.
I can't think of much else right now. Do you have any other specific things you were pondering over?