Sand the wall in small, circular motions with 400-grit sandpaper, and then wipe it down with a damp towel or rags. Use 600-grit sandpaper to polish the surface in the same manner. The more you sand, the more contrast there will be in the color. Remove the dust as before.
You should see the texture between the first layer of plaster and the second. If that isn't shiny enough, hold the trowel almost flat against the wall and polish the surface as above. This generates heat and friction, resulting in a distinctive sheen.