The xenofex filter arallegro's talking about is okay. But you get random lightning patterns. What I'd do is draw a lightning bolt in the shape ( with a black fill) i like in a vector program like coreldraw, export it as tiff, open it in photoshop, copy it to my target image as a new layer, refill it in he color I want, add blurs, and basically fiddle around with it's opacity and maybe even add new layer copies of it for different effects. Play around with the layer blending options until you get the effect you want. Then, I go in with the dodge and burn tool on the background image to simulate the lightning's light effects on the clouds. It's pretty tedious but this way, you get the exact lightning streak you'd imagine. But then again, if random streaks are what you need, go the simpler route and get xenofex's filter. By rendering it on different layers first, you could probably get a streak that would serve your purpose. hope this could be of help.