An easy way to keep a spare key on the outside of a vehicle.
Spare Key (preferably with Key Ring)
Strong Magnet (old hard drive magnets are ideal)
Step 1: Thread Key on to Magnet
Put the key ring through the hole on the magnet.
Step 2: Wrap With Electrical Tape
Get the key flat against the magnet and wrap with electrical tape. This is to prevent the key from flapping around, and also to protect the bumps and ridges of the key from dirt and wear.
Step 3: Stick It to Your Car
Put the key and magnet somewhere on the under carriage of your car. I tend to stick mine somewhere around the rear bumper.
Never get locked out of your car again!