Print out these letters and it will for the sign, Every student in __ period completed his/her homework for __ days in a row. You simple write the period(s) you want to use and then everyday you update the number of days.
Students love the visual, especially since I display this in the hallway.
I began with a simple reward for 1 day, then a bigger reward for 5 days, and so on.
The sign is a great motivator for students to turn in their assignment so they don't let their peers down and since they get positive reinforcement.