Depends. Usually best done at modeling time, by how modeling is done (e.g. extrude along a spline, make sure mapping is enabled when doing so). Failing that, you can just put planar UV on it, and paint the texture in the related areas in Photoshop. Or for some effects, like bump, a procedural may work fine with planar if it doesn't have to "follow the grain" ;) Or for items like STOP written on it, use Decals.