This Very Blog has an interesting technique for making modular forests.
trees
Making Trees From Bottle Brushes
Here’s a nice tutorial on making trees from bottle brushes.
Dressing Up Your Tree Bases
The Wargames Table site has a tutorial on dressing up your tree bases
Making Trees Cheaply
Mitch’s Wargaming blog has a tutorial on making cheap trees for your wargaming table. It looks like a lot of work, but if you have more time than money …
Working With Tree Armatures
Here’s a good tutorial on working with tree armatures.