I mainly use two ways - the first is the mass of 3d bryce trees approach for forests that will be viewed up close. There was a thread in here somewhere not so long ago about doing it that way or pointing to a tutorial about doing that. The second way is the mass of 2d picture objects (with each picture being a tree or two or three). I use that for forests that will be viewed at some distance. (example -> http://www.homestead.com/syndre80/files/siegeA_FINAL.jpg) For forests that are to be viewed at an intermediate distance, I tend to use a combination of 2d and 3d trees - the 3d trees are placed so as to break up the planar/flat look of the 2d trees.

Dreams are just nightmares on prozac...
Digital WasteLanD