I came up with a lottery in order to reward homework completion in a tangible way. I have all my students' names on slips of paper. Whenever there is a quiz, I check back over the past five days and those students who have completed all the assignments in that time get their names pulled. From those names, as the students wait for the quiz, I dramatically select a name and that student doesn't have to take the quiz. Homework completion has shot up in all my classes and so have grades.