I place textures over the renders and erase them over the main part but leave them over lapping the render's edges and set it to lighten, screen, darken or something like that. Some people like to erase the edges of the render to blend it in a little, whilst others use gradient maps and lights to blend.
If you look at some of the PSDs posted, you'll notice that in the layers the render is usally near the bottom, with textures and stocks gradually placed over them.