The 1000 Foot General has an article on improving the looks of standard model railroad trees.
Making Large Removable Canopy Stands Of Trees
For areas of particularly dense forests, creating large tree bases with removable canopies allows you to properly hide your figures.
Making Modular Forests
This Very Blog has an interesting technique for making modular forests.
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