Bring a fire extinguisher, although you may already know that being in local shows. Also, any ppwk for your car, like the build sheet, owners manual, brochure. I use Meg's Quik Detailer when I get to a show to get off bugs/heavy dirt, and a cal car duster for light road dust. I also do the interior once I get there, but that's because it's a vert (road dust gets everywhere with the top down! buckle the seat belts, I remember that's points at bigger shows-although I've never done a bigger, MCA show. Verts are supposed to show with the top up too, and I believe you are supposed to roll up your windows as well, in all cars.
In my show detail bag (this is for the 00 btw, the 73 isnt err "show ready" yet, although we plan on entering a few shows with unfinished/resto in progress classes this year):
Quick detailer
Megs interior cleaner wipes
Megs interior protectant wipes (the armorall ones leave too much lint, imo)
Window cleaner wipes
Tire gel (megs endurance)
Gold class trim detailer (for the black plastic trim)
lint roller (gotta thank a Megs forum member for this one, I tried it on the trunk fabric and it works real well to get little pieces of dirt off)
SUNSCREEN (people always forget this one-it's usually a good 6 hours outdoors in the sun per show)
Towels-lots 7 lots of towels. I'm a recent Microfiber convert, so I think I have about 15 of those, plus some terry for stuff like the tires.
Meg's clear plastic polish
I also bring a cal car duster, show chairs (we have the ultimate chairs-they're rockers!), a copy of my build sheet (cant risk damaging/losing the original!), a brochure for 2000, an owners manual, a copy of the 30th anniv Mustang Monopoly, a fire extinguisher painted the color of my car (a winter project 2 years ago), and all my dash plaques & other awards-oh, and a small hotwheels of my car (I lucked out-my husband found a "then & now" johnny lightning set with an electric green vert w/a parchment interior in it-bought 2, one to keep and one for display). I put the hot wheels up high on my intake manifold, out of reach of small hands.
When I get to a show, I dust, then QD (for me, this includes under the car). Then I do all the black trim so it looks nice & black. Then the interior. Shake out the floor mats. Organise the trunk (that's where I display the extinguisher, and ppwk). If I brought it, I put out my "story board" (display board with the copy of build sheet, article when it was the "Mustang of the Month" in our club, dash plaques, and a short bit about where/when we bough the car, the build date, and production #'s for 2000, broken down to total production, # of verts built, and # of e-greens built). Then all the clear plastic (taillights, etc) get done. Lastly, I do the tires (cause that stuff is greasy. I use a cleaner wipe to clean my hands, but it still leaves residue I don't want on my paint!). Then set up the chairs, put on some more sunscreen and get ready for the fun!
As far as preshow prep, I found a new combo I like for paint care. I use a claybar, then paint cleaner, then polish, then NXT. Let the NXT cure for a day or so, top with Gold Class wax (all Meguiars).
My best suggestion is to clean everything you can reach. My car isnt a dd, but it's first year was as a rental in Florida. It isnt perfect, but it's not dinged up. We drive to shows, we cruise the car, we have gone to events as far as 400(ish) miles with it. But I spend alot of time cleaning it. Last year, I shocked the heck out of a judge when he asked if we drove it-and he found out that it had @ 26,000 miles, and it was an ex rental. He said he was surpised to find out it was actually driven because it was so clean.
So there ya go, hope that helps!