Well, I've seen: Tossed in a box, didn't like that one. Hanging on nails next to the indoor, didn't like that one as they got covered with dust and icky. Hanging on nails in a tack/storage room, meh, OK. Color coded, tossed in a couple of boxes, and sitting on a shelf (these were actually hard hats for something else)
Helmets were always one of the first things that you bought for yourself if you were planning to ride more than half a dozen lessons. Those inexpensive Troxels work out to pretty cheap per lesson once you get over 10 lessons.
Ok, here is how we do it here. You know those tack-cleaning hooks? They are like $3.95 and you can get them in chrome or plastic-coated colors and 2 or 4 prongs?
I was also once wondering what to do with over a dozen "loaner" helmets and had a bunch of these tack cleaning hooks laying around. Some were 2 prong and most 4 prong. I threw some chain over a beam in the barn (you could just also put a hook eye in the ceiling) and and with a double sided clip, hung the first tack hook... then hung another on the first and kept going until I was like 4 feet from the floor. I think mine had like 7 layers total. Since you are only using 1 of the 4 prongs on the "links" on this "chain" of tack hooks, that left 3 hooks every level on which to hang a helmet. It was a nice compact hanging column of helmets. I hung mine at the exit of the grooming area and right on the way to the indoor. On the wall right next to the hanging column of helmets I also hung one of the semi-cirular wooden whip racks.... so the last thing you did after grooming and getting ready for a lesson and you were on your way to the arena is grab a helmet and, if needed, a crop. The smaller, little kid helmets were put at the bottom to be in reach of the kids, the adult helmets were up higher.
The helmets did get dusty, but we really only want people to use these "loaners" for their first couple of lessons or if they totally forgot their own. First off because nothing fits as well as or is a safe as a helmet that fits and is adjusted to your personal head and secondly for liability purposes...
I also have seen that a couple of places have nice shelves that helmets are on in a row and on the edge of the shelf... the size of the helmet is written.