What I would suggest is getting graphite oil.Put it on the key,then unlock and lock the doors.You can also use stuff like WD-40,Liquid Wrench,or Ballistol.
(IMPROVE) Graphite can be EVIL for locks because some folks put to much in. Don't use petrolium products like WD-40 and such as a lube as it dries to a sticky substance and attracts/holds on to dust and dirt. It works well to get a lock turning though. Eventually the springs might get clogged. Use a teflon or silicone base product or something like Tri-flow with PTFE.