Here’s a good photo tutorial on modeling snow and ice. The key ingredient is something called Snow-Tex, which is available at craft stores and works a bit like spackle.
Modeling
Making Forest Bases
I like this technique for making bases for your washer-mounted trees. As it is now, I just put the washer mounted trees on a bit of irregularly cut felt.
Making Bases With Bombed Trees
Here’s a photo tutorial on making bases with bombed out trees. They’d look terrific on a WWI or post apocalyptic table. I think they’d even look good as terrain in Mordor.