InDesign lets you control the visibility of layers in your placed Photoshop graphics. That can be pretty handy when you find yourself designing a series of related ads that require minor changes to the same image.
In InDesign, select a placed Photoshop graphic and choose Object > Object Layer Options. This gives you a dialog box where you can toggle the eyeball next to any layer to turn it on or off.
In this example, a photograph of clam chowder was used for a variety of ads. Some layouts needed a yellow background at the top half of the page, while others needed the existing background. An extra spoon was added for some versions. Using this trick, a single layered Photoshop file was all that was needed.
Here is how those layers appeared in the original Photoshop file.