It depend what style of forest you want.
here, a couple of tut:
http://www.cartographersguild.com/tu...-tutorial.html
http://www.cartographersguild.com/tu...photoshop.html
http://www.cartographersguild.com/tu...otoshop-3.html
http://www.cartographersguild.com/tu...-tricks-6.html

you can also use special brushes: like this one
http://www.cartographersguild.com/ma...-patterns.html

Or use theething
http://www.cartographersguild.com/ma...treething.html