Making eye-contact with audience is a time-tested and powerful trick to engage them while speaking to them. The moment you see someone in his eyes, he automatically thinks he has to respond to you through his eyes and you need to be responded then and there. And just by pulling him to respond to you, you pull him towards your topic and your presentation.
As you may have a big number of people listening to you, you obviously can’t make eye-contact with everyone in the audience. Pick one person in the audience, look into his eyes and speak straight to him as if you are making your presentation just for him. You don’t have to think of others as all of them definitely must be hearing what you are speaking. Finish one part and move to other with another member of the audience. Likewise, after every few seconds, jump to a different person in the audience with the eye-contact.
This trick serves you with two advantages. Everyone in the audience feels that you are making presentation to him or at least talking to him in one-to-one manner. And secondly, everyone in the audience starts suspecting you that in a short while you may start talking to him straight, so to stay away from an embarrassment, all of them get attentive and listen to you carefully.
A careful listening from the audience is what you want from them for your presentation. And eye-engagement is the craft which augments its chances in order to get your presentation well received by all the audience-members.