You don't need wildly windmilling arms, just some interesting movements. To emphasize a particular idea, feel free to point, draw a line in the air, clap your hands together, wring your hands, pound the lectern or table, or use some other gesture. People will follow any moving target, and seeing a little literal animation from the professor helps them maintain interest. Some gestures also serve as symbolic communicators and memory aids. Raise and lower your voice and change its tone as you talk.
Monotones are death to any lecture.
A clear, sufficiently loud, varied , forceful, confident voice is a handy tool for keeping any audience listening. As far back as the eighteenth century it has been noted that a speaker who appears unconvinced by his own arguments is not likely to convince others. Visual illustrations can be pictures, slides, overhead transparencies, diagrams, charts and graphs, or even lists of key points. The important thing is to provide some visual cues to aid your audience in apprehending the structure and remembering the content of your talk. As students continue to come from an ever more visual upbringing, this kind of illustration continues to gain in importance.