Cross-braces for wooden shelves
Ikea sells the perfect thing http://ikea.com/us/en/catalog/products/87749600/ except it's too short and there's no Ikea in town; I mention it for the example it provides. These http://www.storeroomshelves.com/storeroom_shelves_bracing/storeroom_shelves_bracing.htm look good too, and are cheap, but are 700 miles away.
They don't need to be strong: I anchor the stanchions to the wall; I need them to hold the shelves up when I mantle and dismantle them. They do need to be thin because the shelves are against the wall.