Use a moisture dispersant like WD40. Give the locks a blast using the red probe tube..
until the solvent (kerosene) runs out of the lock.
Now.. WD40 contains kerosene which dissolves away any grease already in the lock.. which is why it is only a temporary solution by itself. Once the kerosene evaporates away after a few days, you've then got nothing to stop the moisture getting back in... so after the WD40 stage ^^ , you have to replace the grease.
While the lock is still wet with the WD40.. get some regular graphite or white (or any) grease and pack it into the lock. Stuff it in with you thumb and work the key so that it gets right into the lock mechanism.
The WD40 will now help.. it will dissolve some of the grease and carry it into the places it needs to be... then after it evaporates away it will leave this new grease behind.
Pack the grease in until the lock is pretty full of it. You have to get grease into the works.. so turn the key to work it in... then leave the key slot with enough grease still in it to keep out any moisture.
Also.. use some tyre shine spray on a cloth and work it over the door rubbers (where they contact the doors). It keeps moisture out of the "pores" of the rubber door seal so that the door doesn't get frozen to the seal.
Finally buy some Isopropy alcohol (Propanol) off fleabay and make a 20% solution with that and your existing brew of screenwasher. Your washer jets will never freeze... and it smells nice..