Add a stroke along the bottom of an area, and make it a lighter colour than the background colour of the area.
This is quite rare, but I've seen it in some cases where the body of the design, like a website, is a bright colour, and seems to be casting light on the navigation area above it. This creates a lighter bottom edge.
An object having a lighter bottom edge is not normal - we're used to objects having darker bottom edges since light usually comes from above. If you use this technique, try to make it obvious why there is a lighter bottom edge.