How are you doing it now? Are you sure you're actually capturing the lightning and it just isn't showing up because it's too dark, or could you be missing it all together?
I don't have a ton of lightning shots, but here's how I've always done it:
Tripod, ISO 100, Manual mode with f/8 & Bulb.
Set the focus where it needs to be, then using a remote open the shutter before the lightning goes off. If nothing happens for about 10-20 seconds, go ahead and close the shutter. Try again. The time between when you close the shutter and when you open it again is when all the the lightning strikes will happen.
If lightning actually dose strike while you have the shutter open, you can go ahead and close it, or you can leave it open a little longer for more ambient exposure & the chance of a second strike.
Figure out how many seconds it takes for the ambient light to start exposing the sky more than you'd like.
That's the limit - if nothing happens after that amount of time, close the shutter.