With my groups who have had behavior problems, I use a checklist at the end of the session (I tell them about it the first day, and keep it in full view during the session). They each have to fill in the yes/no boxes for themselves, but the others have to agree to it. It says things like "I stayed in my seat." "I kept my hands to myself." "I participated in the activity."
If they get all boxes marked "yes," they get an extra sticker for the day. Our stickers add up to prizes after a certain number, so extra stickers are a big deal. If I notice that a child is not adhering to the checklist rules, I stick a yellowpost-it tab on their checklist -- that signals that we have to talk over how they did on that rule. At the end, we talk over the tabs. Sometimes we remove the tabs -- depending on how well they did for the rest of the session -- and sometimes we don't. (That way they are guessing all through the session; they don't lose their sticker early on and figure it won't matter what they do.) This really was effective for me in a very mischievous group of boys showing off for girls.