The easiest that I can think of is use snow scatter flock. GW sells it, although it comes in a tub about half full, which is lame. I wouldn't have any but I won some as a door prize... haha... 8 bucks USD for half a tub of white powder lends itself quite well to crack jokes...
A much more economical alternative is to buy the Woodland Scenics brand (for all things basing, I think, since it's all the same as GW's anyway...) and get one of their giant eight-inch tall shakers of the stuff for about $12 USD.
I've used baking soda in the past, but I'm always a little nervous about putting food products on miniatures. Baking soda is ultimately some odd chemical compound, and snow scatter is just white sawdust. Who knows what baking soda will do to a model's base over time?
Seriously, the woodland scenics range of flocks, foliage and other modelling products is fantastic, and blows GW's away. I use all their stuff, particularly due to the variety of colours, as well as textures and far more for your money.
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