All of my courses always have a strong emphasis on in-class discussions. Even when I taught a course with 70 students last semester, it was still organized around student participation. As much as I'd love to stand in front of a large group of students and drone on for two hours, I realize that this is, probably, the least productive teaching strategy. There are very few people in the world who can absorb information in this way for more than 20 minutes at a time.
My way of promoting active student participation has always been very simple: I assign between 20% and 30% of the final grade for in-class participation, and the problem is solved. Still, there is a number of strategies to help students engage more actively in discussions. An older colleague shared such a trick with me, and it works very well. The trick is very simple: after you ask a question, shut up and stay silent for up to 7 seconds. It's a good idea to count to 7 in your head. It is very difficult to maintain such a long pause, but it has been proven that even a 3-second pause after the question is asked improves student participation to a significant degree.
I tried this method, and it really works.