In other words, at transition points, remind the students of your goals and their responses. Show attention yourself by building on a student's point, by withholding judgment until you have several responses, or by listing the multiple responses on the board and asking the students to group them. At the end of the class, summarize the points they have made and connect them to the original questions posed at the beginning of the class. The point is to allow students to come to their own conclusions, but to help structure and analyze them. You should not be afraid of conflict and insist on reaching a consensus.
Opposition can be very educational.
If students are disagreeing in interpretation or conclusion, but are backing their arguments up, that's the nature of discussion. When the entire class appears to have a goal for the discussion that bears no relation to the one you intended or stated at the beginning of the class, it may be a good idea to listen for a while until you have some idea why they might have settled on their own set of issues rather than accepting the ones you intended. Sometimes, finding out what your students are thinking and how they will respond to a given question is more important than momentary control. John Moore points out that sometimes losing control of a discussion is a good thing, even if somewhat scary. If your students have gone off track not because they don't understand the information or because they have arrived at a legitimate conclusion different than yours, go along with it.