This video describes how to build some terrific looking desert and Martian terrain.
modeling
Making Acacia Trees
WarFactory has a tutorial on making African Acacia Trees.
Basing Figures With Baking Soda
A mix of baking soda and super glue can be used to make finely textured bases.
Making Pine Trees
The All Hell Let Loose Blog has an article on making 6mm pine trees.
Modeling Trees
Here’s some advice on modeling trees from air conditioning filters.