The easiest way is to never lock your car, but we know that is usually not feasible.  I try to make a habit of looking at my keys in my hand before shutting the door.  Just in case I fail to do that, I keep a spare door key in my wallet.  The problem there is that the last time I locked myself out of the work van, the key had been in my wallet so long that it had broke in half.  Figures, huh?

    Some dealerships give you a plastic key when you buy the car.  Most of the time, these keys have not been cut yet so you will need to go to a locksmith to have it cut.  Be careful using these keys though.  Since they are made from plastic, they are very weak and can easily break inside the lock making things much, much worse.  If you have to use one, first warm it up in your hands.  The colder they are, the more brittle and likely to break.  Then very carefully use it in the PASSENGER door.  If the key breaks, it is not near as inconvenient as one broken off in the driver's door.

    The best way is to hide an extra key on the vehicle somewhere.  The magnetic "Hide-A-Key" devices don't work very well since they seem to have fallen off just when you need them.  Most cars made now don't have enough metal for the magnet to hold properly.  There is generally some sort of plastic coating keeping the magnets from contacting the metal.  Without direct contact, the magnets aren't close enough and easily get jarred loose from bumps and such.  I've had too many people tell me they had a "Hide-A-Key" that was there just last week.  A newer "Hide-A-Key" type device uses a pouch that is "velcroed" to a another piece that is "glued" to a plastic part of the car such as in the fender well.  I thought this was going to be a good device and sold a few until a customer told me that it didn't stick very well and had fallen off at the car wash.

    There are a couple options left though.  I recommend using a piece of stiff wire to hang a key under the car.  But, DO NOT attach around a brake line or near any moving parts that could be damaged and reduce your safety.  Be sure to use a piece of stiff wire such as a coat hanger and NOT braided wire as in speaker wire or lamp cord.  Braided wire will not stay twisted and will eventually come off thus losing your key.  You will need to clean the key off before using it in the lock as the key will be dirty and dirt will mess up your lock making it not work correctly.  After unlocking the car, just hang the key back under your vehicle so it will be there the next time you need it.

    Another way to hide an extra key is with an all metal key that has no plastic anywhere on it.  First drill the key ring hole in a metal key out to about a 1/4 inch diameter and put it behind the license plate with one of the license plate screws through it.  If your license plates are held on with plastic screws like many cars today, then you can usually remove the screws with a dime to get to your extra key.