When I get a lock rusted up, I generally get a dry spray lube and hose down the keyhole until the stuff drips out the other side. My poison of choice right now is DuPont teflon+wax lube.

If it's badly rusted you may have to go with oil instead, since the dry lubes aren't much of a solvent and won't clean up an existing mess. I don't like oil in locks because ISTM that it'll just attract dirt.