Usually it's nice if the photoshoot is well organized and someone takes direction on leading the cosplayers.
Also, if you see a cosplayer who is taking a break or standing on the side while their character isn't getting photographed, feel free to ask them for a photograph!
I've been involved in multiple shoots, modeling, and cosplaying for over three years now. :3 I love it.
Yay, organization is the key factor! It's always nice to have some sort of event (e.g. contest, angry-faces) going on as well. But ultimately it's a place for people to relax, look fabulous & have fun!