Hmmm.. well, there are alot of ways to do that.. but I do it like this.. I take the airbrush (Photoshop CS2 has it installed in it, don't know about other versions).. choose a prominent colour from the render, and brush it behind the render a bit and set it to soft-light and over-lay.. and the way I do it, the brushing comes between the render and the background.. ofcourse there's alot to do after you put it that way.. means that you have to do alot to make it look better, but thats how I originally do the lightning.. you can also find lightning brushes on deviantart.. they'll be really helpfull too..!!