Coming from a painting background a rule of thumb taught by many instructors for creating the illusion of depth on a 2D surface is to have the foreground advance by accentuating contrast, color and definition while making the background recede by doing the exact opposite.
I've looked at a lot of big country since I've been surrounded by it living in Texas all my life and I have to say my eyes agrees with this. I just wish landscape photographers would process their images the same way.