I have been making wire trees for N and HO projects for some time. The recent trick I learned from Gordon Gravett's book is to start with long stands of straight wire, not a twisted wire or braid. The results have been excellent. The drawback, is that the trees take a good bit of time to make. But in O scale, one tree covers a lot of ground. See
http://usmrr.blogspot.com/2012/03/upping-my-tree-game.html Wire trees are quite durable, so they are ideal for areas where you might get rough handling. Supertrees and dried weeds will eventually break as you operate your layout or work around them.
Here is a shot showing some wire trees installed on my layout.
Note the big tree just to the right the stack is not finished.
I also plan to try some etched leaves to depict marescent oaks and maybe even pine needle clusters on southern yellow pine. This was a technique I saw on a video from 1935 from the Harvard Forestry museum. The southern yellow pine is a tough nut to model accurately but I am desperate enough to try the etched needle route. This comment applies to big scale. For N Scale, Caspia works well enough.
I paint my own back drops. One reason is so that I can paint the trees on the backdrop to match my model trees. By doing this it makes it very hard to tell which trees are painted 2D and which are 3D. Adding a painted tree on the backdrop is a good way to thicken the forest with less effort than modeling 3D trees.
I also found that branches from my wife's miniature crepe myrtles make good background trees when placed near the backdrop. Her full size crepe myrtles don't have enough small scale ramification to be convincing. The miniature crepe myrtles also have nice scale bark texture.
I am lucky in that most of the trees in my modeled era and area have been cut down. Adding a tree stump is much, much easier than adding a 3D tree. When employed in mass, they convey the feeling of a finished scene with much less work. This photo shows what I mean as you examine the area behind the bridge to the right of the wagon.
