I always only have a temporary layout and I would guess that I've simulated snow 1/2 the time.
I have always used Baking Soda (Arm & Hammer in the yellow box). I have put it on structures and also on "the ground". My "ground" is Homasote that I've painted green. For comparison sake, my completed layout it usually up for 2 months meaning the baking soda is resident for that long too.
I'm guessing here, but my observation makes me think that the problem is that moisture in the air seems to get absorbed into the baking soda and then that sticks to what it's touching.
I didn't have any problems removing it from the Homasote: I shop-vac'd it up and then had to follow that up with a damp rag washing to get the stuck on stuff.
The structures were a little more difficult but I did get all the stuff off - eventually. I tried to blow it off using canned air and that got most. But like I mentioned, it stuck to the roof and some other parts. Those areas needed a very careful damp rag cleaning.
I've also snowed up vehicles and had to follow up the blow off with a damp rag cleaning.
BTW: I do think it has a nice snow look. Some of the glittery/shiny specks that others recommended would help - I've never done that step (out of not knowing).
- walt