Mitch’s Wargaming blog has a tutorial on making cheap trees for your wargaming table. It looks like a lot of work, but if you have more time than money …
Here’s a good tutorial on working with tree armatures.
The Joy and Forgetfullness Blog has a useful post on modeling good looking tree bases.
Matakishi’s Tea House has a tutorial on making scouring pad pine trees. I ran across it while looking for instructions on building scouring pad hedgerows. No luck there yet.