The German leather helmet was designed with an anticipated life of ten years of service use. The fact that they are now prized collectibles several decades after their manufacture was probably not anticipated. So, it's up to you, its current owner, to ensure its longevity and preserve it for future collectors. Here are some basic guidelines for preservation and conservation of spiked helmets. This guide will attempt not to enter into the grey area of cleaning and restoration.
Sunlight is extremely damaging to everything but plants.
Any window that allows direct sunlight to fall on an artifact of any sort must be curtained at the very least. The damage potential of sunlight also applies to fluorescent lighting. These lights, although universally used, emit UV rays similar to sunlight. In any room where collectibles are displayed with fluorescent lighting, UV filters absolutely must be placed over the bulbs or severe fading can occur. UV filters are not expensive, but they are very difficult to locate.