This is a good question- one that I spend a lot of time thinking about, as forums are a very important part of my online classes. First of all, I put a lot of thought into the questions that I post in the forums. They are always relevant to the topic of the week, but they bring out elements not covered in the lessons. The questions are very open ended, and are usually so interesting that students normally do check back in to see what other students have posted. For example, I teach an online Intro to Computers class, and one of the discussion questions this week is, "How does modern digital image-processing technology affect the reliability of photographic evidence? How does digital audio technology affect the reliability of sound recordings as evidence? How should our legal system respond to this technology?" Students usually have quite a number of different opinions about this topic. I always participate in the discussions at the end of the week. I monitor the threads in case there are questions directed to me, but I don't post my summary until the end of the week. My students know that in order to receive full credit for participation each week, they have to post well-thought-out responses to the questions. I don't give any credit for "I agree with you" unless new content is provide along with the agreement. The truth is, it isn't hard to know who is participating in a productive way and who isn't. I also weight my grades, and 30% of the the total grade comes from participation in the discussion threads. The students who care about their grades participate.