Incentives to read for seminar/discussion course I'm preparing to teach an introductory course and to teach the content through assigned reading and viewing of A/V materials plus seminar-style discussion. This approach won't work if students don't do the homework. How can I design the course so that they're motivated to do the reading/watching/listening in preparation for discussion?

This is an undergraduate-level course in a continuing/adult education program at a large private university.
Any student that takes a course should be prepared to do all the course work in the way that the instructor lays out the course. 

I had a science course in which I was told "In order to pass this course you will need to study 12 hours between each class" this was a twice a week class. 

The entire course was pretty much homework based and do-it-at-home. The professor didn't need to motivate us to do that work because we knew that we could not easily pass without doing it and we paid a lot of money to take the course in the first place. 

As a student and someone who is finishing up their first course that they are teaching: You can't force someone to do something to help pass the course. You can stress how important it is to do the homework and that it would be a waste of their money if they think they can do well without doing the homework. Remind them of the best practices to use when working on the homework and how long it should take. If you notice a student not being able to keep up talk to them about their homework habits. 

In the course I mentioned above most students who did poorly had a lot of personal responsibilities such as a job or children. The amount of work required outside of class shouldn't be so heavy that people with other responsibilities than school just can't complete it all.