The seminal site Terragenesis has an interesting bit on making alien vegetation from drinking straws.
modeling
Modeling Tree Bases
The Joy and Forgetfullness Blog has a useful post on modeling good looking tree bases.
Modeling A Pig Sty
The Miniatures and Terrain blog has a tutorial on modeling a pig sty
Making A Guard Checkpoint
Here’s a tutorial on making a nice guard checkpoint. It’d be useful for a variety of 20th century settings.
Modeling Sandbags
What’s a modern battlefield without sandbags? Here’s a nice tutorial on making some.