Eye to eye contact with audience with modulation of voice will make your Extempore Speech interesting and you will be heard with attention.
Positively your domain knowledge, focus on topic empowered with communication will be essentials to be judged at extempore speech test by selection panel. But other attributes of non verbal communication will lead you to be at ease with your speech. A bit of theatrical suspense will also add pinch of salt in your speech which will be liked by audience but ensure not to go out of focus from topic.
With moving your eyes on each one of your listeners give the impression as if you are talking to each one of them and they equally will respond with their face expressions which you can read and lead yourself in direction of topic. This will also help you to conclude the speech in most effective manner with take away from you as speaker.
Although there are many ways to prepare for your speech but best would be to think and relate with some practical ideas so to be successful at Extempore Speech.