There are a variety of containers specially designed to "hide" a key on you car. However, they are all subject to discovery. They also mostly rely on either magnetic or hook-and-loop attachment which can be accidentally dislodged.  One of the more innovative designs around hides a spare key in the gas cap. (This works well only if you keep the gas key separated from the door key.) 
If you do hide a key in your car be inventive in your attachment method and key location. Keys in the wheel wells, under the bumper and behind the license plate are all well known places. - Updated: April 22, 2002 

When do I need to change locks?  

There are two main scenarios which point to changing of a vehicle's locks.
1) If you have just recently purchased the vehicle used.  Who knows how many keys are out there and who has them.  Paying to get the car on a new key is a lot less suffering than having the car taken! 
2) If the existing keys are worn and don't work right you may need to replace the locks.  Remember, only half of the wear happens to the key, the other half happens to the lock.  If your key is worn the lock mechanism probably is too. - Updated: January 11, 2002

Can you make my whole car work on one key?  

The answer is generally no.  If your car didn't come with a single key system it usually cannot be retrofitted to be that way. - Updated: January 11, 2002

Can my ignition key be duplicated?  

Yes, the ignition key can be duplicated. However, if you have a built-in security function the key may have to be duplicated by a car dealer.  A call to a local lock shop should quickly determine if they can do the job. - Updated: January 11, 2002