Graphite comes in an applicator bottle put open end in the key hole and squeeze bottle slightly. This can be purchased at any hardware store and is easily applied. Graphite is messy so use with caution, it will come out of the container resembling dried oil. After this application put the key in the lock and work it in and out when it is loose proceed to turn the lock. One should lubricate locks annually or as needed to insure sticking does not happen at all. Lubricating the bolt as well as the lock will help it work freely and dependably.
Check screws in the locks from time to time to ensure they are there and are tight. If a screw is missing replace it by using the recommended screws for the lock and only pan head screws on the latch side of door. Maintaining locks in this fashion will give a person piece of mind and keep one from being locked out on a rainy night.