Ryan Scow has a tutorial on making miniature wargames trees from sage brush. They are very good looking.
Ryan Scow has a tutorial on how to make balsa strips look like timbers for use in models of half timbered houses, stairs and other miniature wargames terrain projects.
GHQ, maker of those fabulous microscale armor and ships has a tutorial on making ocean hex tiles. The techniques also could be applied to other terain forms, such as those large styrofoam squares so many seem to use.
WarFactory has a tutorial on making African Acacia Trees.