Lectures, as noted, need to be limited to about 15 minutes to keep the students' eyes from glazing over. However, many topics require expositioins longer than 15 minutes. One way I have found to break up a lengthy lecture is to periodically ask questions of the class. If I have just made a point, I might ask if anyone can think of an example from their own experience. Or, if there is a connection to something previously learned, I might ask them if "this sounds familiar?". Or I might probe for rationale by asking "why do you think this is done"? By doing this, other voices are heard. And student minds shift from passivity to activity.