This is a particular technique that I have found to be very useful. Painting lightning arc freehand.
It is far more useful a technique than I had expected. It is terribly useful for painting on Space Marine power weapons or for adding some splashy effects onto Cygnaran weapons.
Also, if the same effect were painted in orange rather than blue, it would look less like spider lightning and more like cracked earth lava effects (good for Salamanders and Balrogs).
These gun mages show the details of the lightning pretty well. You start out with a completely black area (highlighting the area up isn't really necessary, and might even conflict with the freehand if you're not careful).
First, paint on the lines of lightning with a slightly watered down turquoise. Make sure the lightning is branching out in the right directions. These lines can be a little thick, don't worry about it. For realistic lightning, remember that lightning usually branches out as it travels downward. It is best to start at the top and draw the lines down the mini.
The next step is to go over the center of the turquoise lines with mix of turquoise and white. This lighter mix should go over the center of each line, allowing the edges to still show your darker turquoise, making the lines look like they have a faint glow about them.
The final step is to simply put dots of pure white into every intersection of the lightning. Lightening up the intersections will make the whole design fill come more alive.
The process for the gun mages is almost exactly the same as the process as used on weapons. The only real difference is that the lightning should travel down the weapon from point of impact backwards. For example, lightning shoudl travel from the tip of the hammer back, or from the tip of a sword to the hilt.
Often on weapons, you're working with a smaller area, so you have to make sure that your lightning arcs out without overwelming the weapon. This isn't too difficult to do, since you can always simply clean up any troublesome lines by adding in more black.
That's about all there is to it. Happy painting.