Pink and Yellow slips can be used to reinforce both positive and negative classroom behavior (fantastic idea I got from Rick Morris!). I use them in conjunction with the Expectations and Contract (see My Products).
The Pink Slips lists classroom expectations (stay on task, be on time, follow directions, etc) and consequences (warning, call home, detention, etc).
The Yellow Slip lists the same expectations with rewards (name on Thank You Board, positive phone call home, 10 minute free time pass, etc).
Both the Pink and Yellow Slips allow for students to check a box on the bottom and write the teacher a message on the back. This helps to avoid disruptive arguments during class. If a student was late or having a bad day they can explain their behavior on the back of the slip before turning it back in.
I have students complete the slip (name, date, period and seat #) and turn it back in to me. I then file them by seat number to keep track of how many each student receives.