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.
The 1000 Foot General has an article on improving the looks of standard model railroad trees.
For areas of particularly dense forests, creating large tree bases with removable canopies allows you to properly hide your figures.
This Very Blog has an interesting technique for making modular forests.
Here’s a nice tutorial on making trees from bottle brushes.