Plan questions that will produce a number of successful responses at the start of the class period. Encouraged, students will be more willing to participate when the going gets tougher. When energy starts to flag, move away from passivity. Don't tell students when you can ask them how. Have students introduce, summarize, or report.
Variety can recapture student attention.
Guest speakers, panels, debates, or videos change the face in the front of the room. Games, groups, field trips, demonstrations, or activities can break students out of the routine. There are many factors that shape student motivation. Ask the CTL for a chapter of tips for motivating students. A teacher's enthusiasm for their subject is infectious.