In this video, Deke McClelland shows you how to create a base for realistic lightning which you can then tweak for your own purposes. You will need to make an account at his site to access the video, but its all free.
http://www.deke.com/content/dekes-techniques-62-making-synthetic-lightning-photoshop
http://www.deke.com/content/dekes-techniques-62-making-synthetic-lightning-photoshop