For the break in period I would just be careful of the material that you watch. If you are watching 16:9 TV without still images that are on the screen for long periods of time then it should be fine. For 2.35:1 movies and 4:3 content, just try to limit them as much as you can to be on the safe side. After that you don't have to be as careful.

I wasn't as careful when I first got my plasma and there is a subtle difference in black levels that shows up in the middle of the screen during really dark scenes. I played a racing game for 3 to 4 hours at a time during the break in period and I think that is what the subtle difference is blacks is from.