In this landscape Joyce Hicks makes sure to separate shapes everywhere there is a step back into the illusory space. Notice that she keeps a hard edge between shapes. How is she using color to suggest depth? What about value? And what is she doing with placement of the shapes relative to each other?
Camilo Huescar uses a different balance of variables to suggest space in his landscape. It still involves value, color edge quality and composition, but the "settings" on his dials are not the same as Joyce Hicks'.
It is often useful to exaggerate one kind of difference between shapes in order to downplay another.
Piet Lap maintains a hard edge between water and land all the way back in the space he describes, which ensures that we will still feel the space even though he has allowed the edges between shapes above the waterline to blur together. Is this deliberate or accidental?
It is not difficult to see which shapes are in front of which in this photo, but there could be confusion in a painted version. That roof, for example, hard-edged and very light, would jump forward in space. What could you do to keep it back where it belongs? If you wanted two tiers of foliage behind the building, how could you adjust color, value and edges to realize that transition?
For homework, either copy one of the paintings, above, or paint a version of one of these photos. Pay attention to the variables. Keep track of how you decide to dial them up or down to create the illusion you want, so you can tell us what you've done. Remember, there's no sin in making it really easy for the viewer to read the illusion. Or in making it deliberately ambiguous. You can move shapes around for better overlap. You can soften hard edges, neutralize some colors while intensifying others, lighten this, darken that. Question everything! Just because the shadow on the tree on the right is the same value and color as the closer one on the left doesn't mean you're obliged to paint it like that. Change it till it works. You can do whatever you want!
And have fun.