Definitive woRKFLOW FOR lANDSCAPES.

This is one of the most comprehensive workflow Tutorials for Landscapes in blender.  He talks about how to blend textures, use meshes instead of displacement, and using lattice for randomness.   If you watch this video 3 times, first take the info in.  Second, take notes, third, Try different parts using his Add on. (free version)

You can also view Martin's ideas Also:

And ALso mentioned in the first video above: (free links to heightmaps) snowy landscape

He shows his workflow also.  Using proxy models is the big takeaway for such large scenes.