Tom Latvys (the Blender Tutor) demonstrates how to set up and layer textures when rendering with Blender Cycles, and shows how to essentially add decals using Blender Cycles Rendering Engine by multiple UV maps on a model and using multiple shaders to display different materials.

In this Quick Tip about layering textures, I go over how to use multiple UV Maps on your model to add a base texture, and then add a second (or more) texture on top of that one

With this example, Tom shows how to create some graffiti on a brick wall using multiple UV maps and also shows how to apply separate shaders to different textures all in the same material. Check out the tutorial for How to Layer Textures in Blender Cycles over at BlenderTutor here.