Suggestion #1 - Buy some:

http://www.renderosity.com/mod/bcs/flinks-conifer/90317

http://www.runtimedna.com/RDNA-Old-Wood-Vol-4-Snowy-Pines.html

Suggestion #2:  Find a lower poly tree such as the one at Planit3D or a Bryce preset (hold down ALT key then select tree icon).  Duplicate the needles of the tree, move them up slightly and give them a white snowy texture.  (I've done this.  Looks sort of more like heavy frost than snow...)

Suggestion #3:  Take a tree mesh into Wings3D, Hexagon, or other modeling software, duplicate the foliage, move it up a little, and enlarge it into a blobby shape to make snow.

That's about all I can think of!