okay for forests there are two great styles you can use
Pasis' style which can be found in his tut here, the full tut is on page 5
[Award Winner] Rising up the mountains in Photoshop
Or you can use ascension's style here Mini tutorial on photoshop trees
I personally use ascension's style but I've being experimenting a lot with it of late. Both styles are open to experimentation.
also, sorry if i broke any rules for posting links to other people's stuff