I set tab stops to mark the margins for dialogue and such. Once I have a scene the way I want it, I put in my own line breaks to trim the right margins.
I haven't used it yet, but Scripts Pro looks good. And a lot better suited to this particular task than Pages though I do like Pages for most everything else.
Final Draft is STILL coming, but they will alert you from their site if you sign up.
It's just a matter of time. iPad is a perfect writing tool if it only had the apps. Final Draft for iPad is one more program that would nudge me to buy one.