Fat tires, low psi (<15 psi) are key. Generally the fatter the tire, the lower the psi w/o pinch flatting, will improve flotation.
For what it's worth, I settled with one set of tires (Schwalbe Ice Spiker 26x2.1) for winter off-road MTBing as I was too lazy to change tires based on daily changing conditions.
For improved flotation, I did experiment with fatter tires (2.5@10-15psi) but found that the tire pressure made a bigger difference. On snowmobile tracks, my Ice Spikers would sink @30psi but float @15-20psi. The fatter tires would float a little better at the expense of the studs. The studs provided much better traction on ice and hardpacked snow trails.