Here is a pretty neat guide for shooting lightning that I cut and pasted from another website. Looks to be very informative:
"first........make sure you have a tripod. secondly, you won't have much luck during the day, so make sure you go out after it's completely dark. I've not ever had much luck with any daylight out. Find a "safe" spot to shoot (there may not be many so be careful...this can be quite dangerous!) and set up before the storm hits so you're not out in the rain (may want to take an umbrella). I prefer to take someone with me just in case something happens. Just as a reminder, you do NOT want to stand out after it's raining as you will get water everywhere on the camera...even with an umbrella, and again, please be careful as this can be very dangerous. Don't stand outside if the storm gets too close. My wife knows a guy who did get hit by lightning a few years ago and he did recover, but he has tons of problems with motor skills, and other problems as well.
As far as camera settings, you will want to use "manual" mode and probably set your shutter speed to "bulb", aperture to f 10 or so, and use ISO 200 (that's what I usually do). You can always experiment with this. You also may want to invest in a remote release for the 20d, I have one and it's 10 times better than pressing the button and holding it down on the camera. You can't use autofocus, turn it to manual and focus just shy of infinity. I zoom all the way out, you may want to use somewhat of a wide angle lens. Aim where you think the storm is coming from and depress the shutter button (or the release cable) and hold it down. Wait until lightning hits and then you can let go. That will expose the shot. The lightning will have plenty of light, it's almost like daylight out if it's close enough. Also, if you have the noise reduction turned on in the camera settings it basically takes double the time to take the picture (i.e. if you expose the shot for 45 seconds, it will take another 45 seconds or so for the camera to process the file and reduce the noise. During this time you won't be able to take another picture) Anyway, the actual pictures will be dependent on obviously where the lightning is hitting (It gets depressing when you have your camera shutter open and the lightning hits just off to one side of the frame!). Overall just be patient and try to take your time in trying to find a good spot and BE SAFE!"
