Wembley and Old Trafford are the only places I think I've actually been in front of my seat. Maybe it's because most of the northern games I get to aren't completely sold out - but no-one seems to bother looking at their tickets when walking up the steps, and all gravitate to where they want...
It often gets mentioned that a dedicated 'singing section' would help. But basically, a seat free-for-all would maybe be the best thing to help create this naturally - which is probably one reason away games are noisier. And also, it would mean that people have to get into the stands a little...