When we had battery problems with vehicles in the Bosnian winter we used foam (rob it from engine bays in scrapyards) to insulate the batteries, also park out of the wind if possible and where you can, park the vehicles very very close together so some are sheltered from the chilling effect of the wind*, (if you can get one going you can normally tow start the others

) also take a leaf out of the Panzer crews books and run the engine regularly up to temp and for long enough to give the battery a boost.
Some people also advocate switching on the sidelights for a short while before cranking (in very cold weather) the theory being that the reaction in the battery will generate a little action in there and make the shock of cranking the engine a little less severe... dunno about that tip myself, never tried it!
*Mind you, it was cold enough overnight that a 50/50 kerosine/diesel mixture looked like a slush puppy
