A lock is a mechanical device, like any other mechanical device it requires maintenance to continue to work correctly. Lubrication is the single best thing you can do for your lock.
When to lubricate a lock
If the lock is exposed to the weather, like the lock on your front door, or the door lock on your car, you should lubricate it about every 6 months, and after a large rain storm. I find that for the first couple of days after a heavy rain storm people begin to have issues with their locks. This is because the rain flushes out the lubrication in the lock and leaves it feeling very stiff, or not working at all. A small amount of the correct lubricant can have most locks working correctly very quickly.
What lubricant should be used on locks? I like Tri-flow; it is sold in most hardware or home improvement stores. I would recommend you do not use powdered graphite. It is acceptable as a lock lubricant in a very limited number of situations. If used in the wrong situation, it can lead to lock failure and costly repairs. Penetrating lubricants like WD40, 3in1, Liquid Wrench, to name the most common, are a bad idea as well. They will attract dust, and be flushed out of the lock more quickly by rain. All lubricants can be useful for the correct application. Using the correct lubricant for your application will keep things working smooth.
Emerald City Locksmith at 1229 W7th Ave, Eugene, OR, 97402.