I usually mix some white paint with PVA glue and add in some modelling snow (like modelling grass), but perhaps white sand works just as well.
For a more icy look I've seen a lot of people using backing soda.
Here's a quick tutorial on how to do this:
BuyPainted - How to make snow base for miniature?…:
http://youtu.be/zsOTFMDVh0g