How should I store fountain pens long-term?

Empty, clean, and dry — that is the entire secret. Flush each pen with water until it runs clear, let it dry disassembled for a day, then store capped, out of sunlight, at ordinary room humidity.

A pen stored inked is a pen you are choosing to maintain; a pen stored clean is inert and will wake up in a decade exactly as it went to sleep. UV is the silent killer of celluloid and resin colors — a drawer beats a sunny display shelf.