If you can't get through 1.5 hours of sitting still, it seems like it's time to consider if law is the correct career. There will be times when you must sit still and behave, etc., for much longer.
As for the prep -- don't. If you're doing a JD/LLM program (I second the "why" -- it's not like an LLM in an expected subject like tax), you're going to have many years of many classes. Enjoy the time you have now.
I also agree w/what thorc's had to say. Briefing will teach you a lot; don't just buy briefs.