I find it important to at least make an attempt to simulate nature, and in pursuit of this, I tend to dirty up my rocks. If there is a an indentation on the rocks, or a crevice or even along the any flat sections on the top, I will add basing material and fallen branches etc. This I think is just beneficial to the overall appearance of the terrain.
When I am doing my plantings, there is normally a tray I am working over, this tray generally has basing material, bits of lichens, a couple small twigs etc in it. This mix is perfect for dirtying up the models. Drop some super glue, a pinch from the working tray, and you have the perfect mix. It will look much more natural than if you try to simulate it. I also do this in a few places on the area terrain as well.
Next I add lichens to any ledges on my rocks, and then hang some from the sides. I do not want them to literally hang much, as they will get caught on models and tear off easy. Instead first glue down the top or highest section of the lichen bunch you are putting on your rocks. Trail some glue down the side, towards the ends of where the lichen will reach, and hold your lichens there for a moment.
Lichens are rather spongy, so they tend to pop back out to look more natural after being glued. This is perfect for the effects I am trying to create.
One special note :
While planting I tend to over use super glue. This is counter inuititive to our normal model building process in the hobby, where we need only a small amount to do a job. For terrain, especially plantings, I tend to use a lot, and this will cause some horrible fumes. Please work in an open area, and open a nearby window.
The last thing I do on the rocks, is add a little bit of greenery on the ledges. The plastic plants work great for this.
I like to create small pieces of terrain as it makes set up and the appearance look better. These small pieces can be sat next to larger pieces, or spaced out to help create a better wilderness look. These pieces are normally about 4" or less in size, and I make boulders or single bush or tree stands out of them.
Now its time to use the medium green lichens for the tree tops. Is this necessary? Yes. For one if you just leave the branches to form barren trees, you will bump them while you move your models through them. The foilage gives them easier to see, so there will be less bumping of the terrain piece. Second, it makes the trees look incredible.
To do this, simply take a clump of lichen, add a good amount of super glue to the end of the sticks, and apply. Push the lichens through to where you want them, and then once again add glue to the top of the lichens where the top of the stick is. Let it drip through on its own. This will help secure the lichen onto that small branch top, and stiffen up the lichens around it providing a tighter grip.
When you have several branch ends near each other, use a single larger clump of lichen. This will give you a bigger canopy above. It will look better, and be more secure down the road than single clumps.
After you have finished with your trees, let them dry a good amount, and then once again, put your models into the terrain to see how it is holding up. Your grasses and bushes should be pretty firm. Accidental bumpings or dropping of the terrain should hold up. ( I have already dropped a couple pieces of my own).
If you have excessive bowing of large pieces of area terrain, you may need to shave off the low points along the bottom to flatten them out again, but if you did a good job pasting both sides, you should not have this problem.
A word of note, my Wife got mad at me for buying a ton of terrain after making this set of terrain. It took some time for me to show her that I made it all from all the parts I have been saving and working on. She wasnt really all that mad, but she did question if I had bought it. (we are buying another house and money is pretty tight until we get settled). I will take that as a compiliment from her on how well the terrain pieces turned out.
If you have any questions please ask. Here are some more pics of the final pieces.