This not-very-obvious technique for lightning originated in a post from Marcos Gabriel to comp.graphics.apps.photoshop. With some modifications, it is extremely versatile, as you will see.

As originally posted, the technique begins with foreground set to white and background to dark blue. Using these colors, render clouds.

(I'll talk you through the basic technique, even though I'm showing a variation that I'll explain later. You may see different colors; that's OK. Just keep going. I'll switch you over to my version of this technique in a few minutes; it's much easier to control.)

The next step is to render difference clouds.

The next two steps are to solarize (which in many cases will not make much visible difference in the image, if any) and from the Image menu, adjust -> equalize.

Again from the Image menu, adjust -> invert.

We have lightning!

Some clean-up (credit to Rick Nelson) will result in more realistic lightning--which, after all, is rarely as fuzzy as the previous step. This clean-up is done with the Image menu again, using adjust -> levels. You will want to move the middle slider way over to the right. It stops doing much when the corresponding value in the dialog box goes to .10, so you may want to click OK on the first try and then do it again.

It's confession time. I haven't exactly done this the way I've told you. Instead, I created a dark blue background, added a layer, filled the layer with white, then did the lightning (exactly as described so far) in a layer mask. If you look at the layers palette here, you'll see more clearly what I'm talking about. Beyond making it easier to control color, you'll soon see why this is so useful.

This isn't really as mean a trick as it seems, because I'm going to teach you a useful trick with layer masks: how to paste into a layer mask! You can take the lightning you've done so far, and keep working with it.

Here's the trick of pasting into a layer mask. Before starting, do a select all of your lightning, then cut (not copy) it to the clipboard. Fill the layer with your background color. Now we need to create a layer with a layer mask, so add a new layer, fill the layer with white, then add the layer mask (layer -> add layer mask -> reveal all). With this layer still active, open the channels palette. The layer mask for the active layer will show up as a channel. Make that channel active, and paste your lightning into it.

If you want to edit the layer mask, make it visible and hide the other channels. When you're done, hide the layer mask channel, make the RGB channel visible, and go back to the layers palette.

This can be a very useful way to edit your lightning, especially if it becomes part of a complex image. Here, you see the results of editing while the mask was open as a channel; I've used the eraser to remove some more artifacts that wouldn't be part of real lightning.

Now for some fun. Go back to the channels palette and create a new channel. Paste your lightning into that. Go back to the layers palette, and select your colored background. Then open the lighting effects filter, and choose the new channel as your texture channel. (This was done with a different lightning sample.)

Now try this. Start a new window, and leave the background white. When you create your layer for lightning, fill the layer with black, and do the lightning in a layer mask. Then create another lightning layer and do it again. (This works because, using the layer mask technique, your layer is transparent except for the lightning.) Not bad for fake marble, is it?

As above, but now using red and stacking up five layers of lightning. It makes a rather nasty tangle of blood vessels, perfect for a pair of bloodshot eyes. In this case, I flattened the red lightning, copied it all to the clipboard, and used "paste inside" to paste into the eyeball areas. By using paste inside, I could move the lightning around in the selection area to find exactly the right part to place behind the iris.

There are other ways to combine lightning into complex images; see my Last Supper for an example.

Now you know how to do lightning. Enjoy.