Planning your layout to have alternating patches of sunlight and shade on the ground can be very helpful for showing depth as space recedes into the distance.
Also, creating those alternating areas of light and shade can help keep your ground plane from becoming ambiguous as it stretches away into the distance.
Atmospheric perspective occurs when particles in the air obscure your vision. This is usually very apparent when there is smoke, dust or fog in the air, but it occurs even on clear days. There's always a certain amount of water in the atmosphere, and as we look at objects that are far away from us, the cumulative effect of all the particles we are looking through obscures those objects. If you can capture that effect in your backgrounds, you will be able to create the feeling of depth. It doesn't have to be that pronounced or noticeable. Even a small amount of effort can create a good feeling of depth.
Because of this atmospheric effect, any landscape will have more contrast in the foreground than in the middle ground, and there will be more contrast in the middle ground than the background. The further things are from the viewer, the less contrast they will have on them.
Here's two examples. In example 1 below, the foreground area has the most contrast. The middle ground area has lass contrast than the foreground, and then the background and sky are the lightest areas. The objects in the distance have the least contrast. It looks correct and there's some (simple) depth to the picture.
The same thing goes for detail. Because of atmospheric perspective, we see a lot more detail in objects that are close to us. The moisture (or smoke, or dust, etc.) in the air obscures the details on objects as they get further away from us. Again, you can use this effect to create depth very simply.
Here's another example to show how this works. In example 1 below, the foreground level has more detail. The middle ground has less detail, and the background level has the least amount of detail.
In example 2, I reversed it so that there's more detail in the background and less detail in the foreground. It looks very graphically confusing and makes no sense.
And, of course, atmospheric perspective affects color as well. Color will always appear more saturated when it's closer to the observer and it becomes less saturated as it gets farther away from the viewer.
Again, all of these might seem like completely obvious observations. But most of the things that I know about drawing are very simple techniques that can be used in very complex and sophisticated ways.
Here is a small selection of examples to illustrate all of these points. More layout material to come...