I made a one-page instruction sheet that goes right on top of all the disassembled parts. At the very top of the page, in very large letters, it says THIS IS A FOLDING BICYCLE. Then it says THERE IS ONLY ONE WAY IT WILL FIT BACK IN THIS SUITCASE, and gives clear directions, including a photo of how it was packed when they opened the suitcase.

Nothing's idiot-proof, but I figure it can't hurt.