Sergio-
My main line of work is modelmaking and prop fabrication and I have done lots of snow effects. Trengove has a number of different products that work, but for closeup work the crystal frost (same stuff as mentioned above in some diapers and as a landscaping additive to retain moisture in soil) is the most convincing, but only if you are looking for a slushy effect. Most mashed potatoes are a little too yellow and coarse for close work, but they are fine for wider shots.
Believe it or not, the most effective look for a finer, more powdery look in snow is salt. I buy it in bulk, sold as "mill feed" salt for animals, and have done shots with it that nobody can believe is not real snow. The mill feed is slightly coarser than table salt. The best part about using salt is that you can get very realistic snowballs, crusty snow over powder, and trails in snow that are difficult with the other plastic snow varieties. Wet it with a spray bottle and it dissolves just enough to compact and allow you to pack it and get a hard shell of crusty snow over powder. I once even did a car shoot for Isuzu troopers that had 12" to 18" of snow on one of their cars. By moistening it, we got a really realistic broken edge just like snow shears off after a blizzard when it piles up on a car. The best part is that I pay less than ten bucks for a 50 pound bag and it doesn't get all over the studio like the plastic kind. The worst part is that is is quite heavy in quantity.
Best of luck,
Ron H.
