Attached Link: Lord of All He Surveys

Rose, you beat me to it.  Scale is the most important element for mountain work IMHO.  Never be afraid to make a terrain say 3000x1500x9000bu to get a set of mountain ridges, then set it back from the camera about 3000+ bu.  You will get great distance mountains, and a feeling of space. 

In my attached pic I have those mountains in the background between 2000 - 5000 bu high and about 10kbu distant.  Because of this, I frequently have images that use tens of thousands of bu in size and distance.  The hard part is often finding the texture balance then.  But you can find it with a little play.

Oh, also it allows you to play with more realistic levels of haze (<10% density), instead of having to pump it up to hide the closeness of the image or give the illusion of space.