Dunno if this is a glitch or if it's a little fun thing the designers intentionally stuck in the game, but this will work with any of the three shields. Make sure you have the Ether Medallion and enough MP to use it. Go to the Dark World, to the area left of the Pyramid. You'll find a house there; the guy inside will sell shields, so if your Mirror shield gets eaten you can come here and buy a new one. You'll see Like-Likes outside the house. Normally you wanna avoid these things, but here's the catch: Equip Ether and let one snag you. It'll steal your shield and try to flee. As soon as it lets you go, cast Ether. The enemies around you, if you left any standing, will all freeze, including the thief that stole your shield. Simply kill the culprit with a good thwack from the Hammer tool, and voila! your shield is back. But look at its blue coloration...it's been frozen! So long as you're on the same screen you got your shield in, it'll stay blue.