Make sure you get the seating arrangement worked out before the flight leaves.

I've heard announcements on international flights that coach passengers are not to enter the business cabin, and that business passengers are not to enter the coach cabin.

I noticed on our recent flight ATL-NRT that there is a new sheer blue curtain that separates business and coach, and that it was snapped to the bulkhead. I was sitting in BE, and wanted to take a walk to the back of the aircraft to break up the monotony of sitting, but realized that that is no longer allowed.