Items floating just in front of the background have a narrow, dark shadow. Those further from the background (closer to the viewer) have lighter, wider shadows. Whether or not objects overlap (but especially when they do) use varied drop shadow size and color to add depth and interest to a layout.
Don't forget to be consistent as well in the direction of the shadow. The yellow boxes all have a drop shadow that falls to the lower right as if the light were coming from a point above and to the left. The upper and lower Near & Far groupings each have their own imaginary light source.