In my classroom, I've always used finger signals for the kids who need to get up and get a pencil, go to the bathroom, etc. In the past, I've just used 1 finger and 2 fingers, but I thought this year I would try to go for all five. It's one of my favorite management techniques. Also, I don't believe in limiting the kids too much on bathroom breaks (I wouldn't want someone telling me I couldn't go when I needed to go!) I can tell pretty quickly which kids like to abuse the privilege, and I always talk to those kids about excessive breaks during independent time turning into 5 minutes working during recess time. It nips that in the bud pretty quick! But most of all, I love not having to verbally acknowledge the kids' needs--they can just get up and go and we can keep going.
I also talk to the kids about times when they CAN'T go: when I am doing a whole-group lesson, when they have an assignment that is being timed, etc. Even then, if they try to go when I'm teaching, I just have to shake my head.
Great little tool, but I've never put up posters before. I created some simple ones to go with my black, white, green, and pink scheme. The pictures are from Discovery Kids.