Obviously the only way to control this damaging action is to store the artifact in a stabilized environment. Eventually the owner becomes perplexed as to why all the visors are loose or lifting up or ripping off entirely. Although the weight of a helmet or cap might seem insignificant, it is extremely stressful on the threads that hold the visor on the body. The weight of the artifact will eventually stretch or break the thread holding the visors on. All headgear should be supported naturally, as it was designed with the shell.
Several options are open here, each with it's own merits.
Many people utilize Styrofoam head forms to hold the helmets. While ideal, I find them large and distracting. Over the years I have utilized empty aluminum cans of various heights for this purpose. Once the can is cleaned, it is sprayed with a dark stone-speckle paint to make them more visually appealing. Then a small round piece of foam is inserted into the open can top, and this is what supports the helmet.