Add extra weight to your car. The key to rear-wheel in the snow is keeping RPMs low and keeping weight in the back. I drive a car that is rear-wheel and my dad and I outfitted it with small moldings that would hold a cinderblock over each wheel well. It was easy to do using simple logic. Other things used to weigh it down are old microwaves, extra bags of sand/rock salt etc.

Of course you want to remove these blocks or heavy items when not in danger of snow to keep fuel efficiency as high as possible.