These are all sound suggestions; I especially agree with the co-GM idea.
I don't have any experience in GMing large groups in tabletop, but I've done it a ton in MMOs. I think the first six suggestions apply to both though. MMOs are probably a better setting for large group RP since everyone mostly controls their own character. I can imagine it would be incredible difficult in PnP.
I've always had a dream of doing some sort of large-scale, coordinated tabletop mega-game which has 3+ separate groups meeting regularly and all playing within the same story. Each group would have its own DM and they would work together so that the stories affect one another. Characters could even travel to other regions by switching to one of the other groups temporarily. Maybe every once in awhile (or possibly in a big climax) everyone could get together for one giant game like the one this video is talking about.
I'm sure someone has done something like this. I seem to recall hearing about World of Darkness having an official campaign which fans could all get in on and affect.