Basic Handle & Lock Maintenance
It's true that over time houses shrink and swell, and incremental changes in their shape develop, usually caused by changing atmospheric conditions. It's difficult to prevent this from happening, but it's possible to make it easy to live with. We do this by maintaining all the items in our homes as much as possible, and the same should be absolutely true of our door furniture.
It's really simple to keep your door handles and locks in great working order, and if you begin to include some easy practices into your broader cleaning regime, your handles and locks will work well for the lifespan of your house. It's as simple as keeping them clean.
Keep it Clean
Locks and door handles don't have many moving parts compared with a large machine like an engine, but they do have one important thing in common: they both have a number of small, rotating parts working in very close proximity with one another. In an engine, these parts will become badly damaged very quickly if they are allowed to rub together without a protective lubricating layer, and the same is true of your door handles and locks. Given enough time and enough use, your door furniture will suffer the same degradation as any other mechanical device. They don't have to tolerate such excessive forces as the moving parts in car engines obviously, but with the amount that ordinary handles and locks are used, it's best to remove as much friction as possible.
You can do this by keeping them well oiled. The quickest way to do this is to spray WD-40 directly into the keyhole, around the latch, along the tube of a deadbolt and around the handle, and then turning the mechanism repeatedly, spreading the lubricant around inside. You only need to do this once a month or so to keep them all in near-new condition.
Remember to lay down an old cloth to protect you carpet before you do this. To ensure the accuracy of the oil, and to make sure it has penetrated the locking mechanism deeply, use the thin plastic straw that comes taped to the side of most spray lubricant cans. This straw can be pushed directly into the hole in the spray cap, and it's a really useful little tool for getting right inside mechanical objects.
Use as much spray lubricant as you can without it pooling in the door furniture. Starting at the top of a lock or handle, you should be able to spray about two or three full seconds worth of oil into a lock. Wipe away any excess oil, and then move onto the next door. Easy. You might even want to oil the hinges at the same time.
Maintaining your Locks & Handles
Another tip: graphite works really well in the lubrication of locksets. Take the key and run an ordinary pencil over it, completely covering its surface (apart from the handle - you'll need to be able to hold the key without graying your fingers). Then insert it into the lock and turn it repeatedly, transferring the graphite from the key to the lockset. This thin layer of lubricating graphite will prolong the life of your locks. Remember to clean the key after you've covered it in graphite though, otherwise it'll transfer itself to your pockets, and then to your hands, and then to everything else that you touch.
If your lockset or door furniture is old (roughly speaking, if your door hardware was installed before 1960, before the sizes and processes of locks were standardised, you can consider it 'old'), then it might need to be dismantled every so often for thorough maintenance.
Each lockset and handle is a little different, but you should be able to take them apart by unscrewing the knob on one side of the door then pulling them away from the shaft. Once you have broken your door hardware down into its constituent parts, thoroughly clean each piece and lubricate them individually, then carefully put them back together. Replace any parts which look substantially damaged or worn, and inspect the entire mechanism for rust.
Outdoor locksets and handles require more aggressive maintenance. They should be fully galvanized to protect them from the rain, and should be sheltered as much as possible from very cold weather. Constant freezing and thawing, freezing and thawing will accelerate the process of deterioration, so keep water out of your locks by covering them, perhaps with a makeshift piece of inner tube nailed over the top to channel water away.
Remember that the best cure for door hardware issues is prevention; if you can maintain your locks effectively then the likelihood of serious problems arising is greatly diminished.