Roofing paper is the base for me.  It allows for unexpected textures, great adhesiveness and its forgiving...  

Then, whatever is laying around, old toys, barbie dolls, scrap wood, pictures, wires, nails, Safari skins, bicycle wheels, more paper, acrylics and oil pastels is fair game...  I don't really search out objects and materials to use, they seem to make themselves present and available...