Simbio.
Couldn't agree more and I commend you on empasizing the positive on the sideline. Complimenting the team on the sideline that is trying to stay in their box is a great technique and works well for me, too.
How about working the chains back 3-4 ft from the line? Success with that? It's worked well for me this year. Others in our assoc fall into the "they'll never do it" mantra. I've insisted on it this year and this has also helped keep the team off the sideline.
I combine this with moving the portable yard markers back to the same distance as the team box. That way I can just tell them to get behind the numbers to clean up and it makes it easier for the players to understand where the box is. (Of course, you get some of those sly dogs that try to move the numbers forward when I'm not looking
)