I taught in an inner-city environment the past two years and had 27-30 kids AND some really rough behavior disorder students in my room. I can tell you what I did. I was pleased with it. I used a pocket chart and gave each student a set of cards. For each infraction, they moved a card. Each card had a greater consequence as they went down the line. The goal was to stay on the top card (a sun because it had a beach theme) all day. It was quick and easy to say "Please move your card" and also sent a quick message that the teacher is not putting up with it! I'm going to use the same system next year. Also, my class was VERY chatty this year, so I implemented a team point system. They sat in groups of four, so I put the numbers 1-7 on a small chart on the board. Each day, I gave points for first group ready with their books out, quietest group during a group activity, people using teamwork, whatever I felt really. I'd simply walk over and pick up the marker and give a point. Secretly, I tried to make sure that nobody got to far behind and that I tried to find a positive reason to give everyone a point at some time. The kids really put pressure on each other to do the right thing...not a bad kind, but they worked better together because of it. Anyway, at the end of the week, the group with the most points got to choose from a small list of rewards that the class came up with. Such as 20 minutes of free computer time, a free no homework pass, 10 extra credit points on the subject of their choice, draw from a treasure chest, or 20 minutes of free play to use the classroom games. Much of these didn't cost me a dime and the kids LOVED it. Getting to play a game while the rest of the class was working was very motivational for them.
sorry to have rambled on. I hope this makes a bit of sense. Good luck!