Tiled Texture Overlays

[3Dski7059]

What technique would work best for adding details to a surface that already has a tiled texture added to it? More specifically, let's say there are random puddles of liquid, or dirt spots that you want to place over a surface that generally has a tile-able texture/material applied to it. Would I have to create a model, that's simply a plane, with the special material/shader applied to it, on top of the floor (wall, ceiling, etc.)? If this planar model approach is the way to go, is there a way to snap it to the surface, as to not have it sink into the floor, wall, etc.?

[xtom]

The decal feature is specifically for this.