Just as the alternator in a car charges its battery, you want your audience to charge YOU. If your audience is a sluggish alternator, you will soon become a drained battery.
Audience members don't know it's their job to charge you. Thus, you have to show them how. You do this by coming out from behind the lectern, asking questions from the get-go, talking in terms of their own interests, and getting them involved in some type of paired or group activity.
Not only does this delightfully raise the energy level, but it solves that dreaded problem of you being "alone up there." When the audience acts as your alternator, you are fully-charged and have comaraderie.