Easiest method is the same method by which I have seen actual snow sleds made to steer and that is to apply 'level' which can be lowered to create drag on the side in which direction on wishes to turn, much like how a tank or other treaded vehicle turns.

The other method by which to accomplish this is, is by multiple runners. Two stationary runners in the back, and a single or double runner in the front that can then be turned/angled to accomplish a turn.

The trick for me is to not think of sand as a thick viscous water, but instead to think of it as snow.