I'm a teacher in training. We had reading "homework" over the summer for our incoming students. Their families were instructed that if their child read every day (15-30 minutes or so), and they kept track of it on a chart we sent them, they would win prizes when they came to school. I think it was something like a pizza party if they read a certain number of days. It wasn't mandatory and there was no punishment for not doing it. I thought it was a great idea.