Gina,
Engaging students can be facilitated with simple techniques to more aggressive methods. Sanding near students while lecturing, circulating around the room, and randomly asking students questions are more simple and often are enough to keep students focused. At times, students lose interest (bored, unprepared, don't understand, are I'll or tired, worried, or apathetic), and these times require effort by the instructor, such as changing to a different activity, taking a break to reset the focus, or changing the method of delivery. Variety in delivery is a great way to keep focus and interest longer.
Barry Westling