Has your key been working great for years but slowly it has become harder to use? You may have to jiggle your key, pull it out slightly while using turning pressure, or put downward or upward pressure on the key to get it to turn?
If you answered yes to any of these questions, and the problem came gradually, the reason you are experiencing this problem is that your key and the pins in your lock have worn down. This may happen faster if the metal your key is made of or the metal the pins in your lock are made of is a harder metal than that the other is crafted out of.
The cheap fix is to take your key to your favorite place with a key copier and instruct them to shim the key up with some folded paper. This will replace the material lost over the years. It may not work as well as the key worked when the lock was first installed because the first pin in the lock will be worn down more than the last pin of the lock, just as the tip of the key will be more worn from wear than the part farthest from the tip. This is because when the key is inserted into the lock the tip of the key is pushed past every pin, and the first pin is forced up by every cut in your key. Therefore, ideally the key would be shimmed more at the tip to account for this uneven wear. It is not an exact thing. Fortunately, most consumer locks are not machined very precisely so a key can be about .015″ off on each cut and still work. I can copy your key to account for this uneven wear pattern.
The proper thing to do is to have a locksmith rekey the lock to match the key you have. The pins’ material should match that of the key. Longest life will be with a key that has the same material as the pins it touches, as anybody who learned about the Mohs hardness scale in middle school science class should realize. Most locksmiths use brass keys and brass pins, but some locks like Medeco or the CX-5 cylinders I sell use steel pins. If you use steel pins and a brass key, the key will wear down prematurely. If one saves the original key and always makes copies of it, then when one key wears out another can be made and the remedy is inexpensive.
If you are going to get your lock rekeyed, you might consider getting all of your locks rekeyed. Who knows where all of the copies of your keys might be?