I also just use a firm "NO" and they seem to get it. We are now at he point where I can call each one of their names and say "go out" and they listen, they stand right by the bedroom door but they are out
. I can say "come in" to each and they will.
I have also banged a wood spoon on a pot and that really catches their attention if they are out of control. I read it here a while back and I have only used it for barking issues (and it totally stopped the compulsive barking fits when someone walks by our door). Sounded a little harsh to me at first but it was actually kinda funny doing it, like a kong. It gets their attention right away.
Good luck!