This time of year its important to keep your locks well lubricated and maintained. With the weather going back and forth from hot to cold, a lot of moisture gets up into the air, settles onto everything, and then freezes again. The water that creeps into your locks can freeze your lock up, preventing you from being able to turn your key, or sometimes even get your key into the lock at all. A quick shot of a good lubricant like WD-40 can prevent the water from getting in at all.
Better to have an ounce of prevention than a pound of problems.