My cooperating teacher had the students line up in 2 lines, one boy line and one girl line. The lines were parallel and she stood to the side, near the back and middle. She would let one line in first, then the other while standing halfway in the hallway and halfway in the classroom. That way if a fight broke out, only half of the class was in the room. This was a school wide system implemented to decrease violence because sometimes students who really wanted to fight would be hiding inside the classrom, something I found out the hard way while subbing after graduation at another school in the district.
Once I did it school, we would get compliments about how patiently and quietly we were waiting, and sometimes how the teacher didn't even know we were waiting to come into their special. Of course that compliment earned us several marbles in the marble jar!
I was always torn about taking marbles out of the jar for whole group misbehavior, especially with the gasp of horror and tears that sometimes followed. I solved that problem with a tip from another teacher - use a marble balance scale. One side is for positive behavior and the other side is for negative behavior. This allows students to see that they can balance out the negative behavior with positive actions without actually losing anything.
Since that time, I've moved onto the clipchart system, which is based off the card system. I love using clipcharts because they show students that there is room for improvement, I don't just focus my attention on negative behaviors, and the system doesn't just center around loss (aka they start at one color and can only go down for misbehavior and without incentive to improve).
Now when we have fire or lock-down drills and the kids have to remain quiet for a long period of time, I use silent gestures to Head, Shoulders, Knees and Toes, The Macarena, The Chicken Dance, and sign language to keep them busy! That positive side of the balance scale is always a lot heavier than the negative side and it's a great math lesson too!