I sent this note via the online course software to my undergraduate class just now. If you see a way to improve on this approach, I'd be pleased to hear.
Class members, this is a quick reminder that we will sometimes have a reading quiz at the start of class. You know from past experience how much better a course is when people have done the reading ahead of time and given it some thought, and that is what a quiz might ask you about: basic details that a careful reader will have noticed, along with an opportunity to show that you have thought about the reading--maybe connected it in your mind to previous readings, previous class conversations, or things you know from other classes and readings or from life experience. All of those are great ways to prepare to contribute well to class discussion.
Because that kind of serious homework preparation hugely changes the quality of a college course, I ask you to do it. In my experience, the conversations are deeper and many more people speak up, and both of those things are great to see. If you have suggestions about other ways to strengthen the preparation for class and the class discussions themselves, please let me know. I look forward to our conversations.
I also usually ask undergraduates to evaluate their preparation and participation from time to time during the semester.