Further to my previous discussions, another way to indicate depth is to include something in the picture that the viewer will understand as being very three dimensional.
Here is a good example. It is Sir Andrew Lloyd Webber's front drive, and you will recognise the depth cue because I'm sure it looks a lot like your own front drive.
In this case the impression of depth is enhanced by the use of a wide angle lens, by the receding scale of the leaves and trees and finally by the morning mist that mutes the distance towards grey. In fact this picture has most of the depth cues that nature offers.