I bought magnetic "Hide-A-Keys":
http://www.amazon.com/Magnetic-Heavy-Duty-Hide-A-Key-Holder/dp/B002O71KXS
from a local locksmith
and position them on the steel vehicle frame behind the grill or inside a steel bumper. Of course I bought them AFTER needing them once: a consequence of not following my dad's example. They have since saved my and my kid's bacon on more than one occasion. They can usually be positioned on a more or less horizontal surface; the magnets are very strong.

It wouldn't be a bad idea to put in a house-key as well.

I like the duct-tape-behind-the-license-plate idea: cheap. works, creative. There's not a problem in our area with plates being stolen. Theft resistant license plate screws are means to make a thief move on to easier pickins: http://www.platelockers.com/