When you need to trash your normal cards, you would shred or cut them. However, your shredder or scissor techniques fail on the nice metal card.
You do not actually have to go as far as melting down your card as long as you take care of the 5 areas that contain private info:
Name on Front and Name and Number on Back
We deal with these one at a time.
Place your thumb on the Back of the card right behind the chip (which is on the front). Push through the card until the chip starts coming off. Peel the chip off. Cut it with scissors and discard.
The Name on the Front and the Name and Numbers on the back are usually raised letters.
Sandpaper them down until you cannot see them
The Magnetic Strip
You need to demagnatize it. Take a refrigerator magnet put it on the strip for 5 minutes (the longer exposure does make a difference). Then swipe it across a few times for good measure.
The only worry with that, someone seing your signature. Use a permanent marker to make it unreadable.