How do you make realistic snow

[Steve B]

I am planning on making part of the layout a mountain region, and with it i fancy some snow, but how to make authentic looking snow is the problem, i have never done snow before. Woodland scenics sell it but have any of you used it, What else can you use to make some nice snow banks by the track side, i thought i could use white polystyrene cut to shape and covered in artificial snow.

any help please, pics would help

Thanks