Set you camera on a tripod. Set exposure to 'manual'. Lock the shutter open. Adjust the aperture according to preference after you capture the first lightning strike. I would also use manual focus set to infinity. Of course, if you are trying to capture a lightning strike during freefall you'll just have to be really, really fast.
My best result a couple of nights ago on my 10D was a 2 second exposure (Shutter Priority/Tv), ISO100, and firing the shutter as soon as I saw a flash. This worked very well considering I was pretty close to the event, and there was a ton of ambient urban light.
I know some folks who have had great results with 10-15 second exposures when they're further away from the storm and in less lit, rural areas. My best 10 second exposure had more action, but ended up looking washed out in comparison.
I also used manual focus, set just shy of infinity. I took a test shot and reviewed it on my laptop to ensure clarity prior to going continuing the shoot.