Better than what my calculus labs did - Assigned 30 questions and then randomly marked five of them (same five for everybody) on everyone's paper. Thing is - you never knew which five they were going to look at.
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.
