How to back up video footage? Experience in on-site data management

The biggest fear on a video shoot isn't whether the equipment is expensive, but that lost footage cannot be recovered. I once had a small commercial shoot where the card wasn't broken, but an assistant mixed up the used cards with the empty ones, and I almost formatted them. Since then, I haven't taken footage management lightly. Now, I divide cards into three boxes: "Unused," "Used," and…