French wire keepers are popular for keeping French hook ear wires on earring display cards, and can be used as earring backs for both posts and ear wires. They also are perfect for holding pendants and other decorations on changeable post findings. But their true raison d'etre is to keep you from losing your favorite French wire earrings. These are inexpensive enough to give away with every pair of French wire earrings that you create.
Another option is SmartBeads" or Silicone-lined bead stoppers. They cost a bit more, but sometimes a metal stopper is the right look.