One of my undergrad math profs went one better on that. She assigned homework due every class period. At the start of class she'd flip a coin to determine whether or not to collect the homework. If she did collect it, she'd flip a coin for each problem to determine whether or not that problem would be graded. The result was that the students had to do all the homework (unless they felt like gambling), but she had to do only 25% of the grading.