The easiest way to regain the audience’s attention is to briefly recap what was discussed and then move on to the next point on the agenda.
If you’re in a large group setting, you can appoint someone to be the group moderator. Their responsibility will be to signal when it is time for a break, when it is time to return from a break, and to quiet the audience down so you can begin. This is especially important if you’re dealing with a crowd of 300 people or more. This allows the moderator to be the person who pulls the crowd in and allows you to stay focused on delivering your expertise.
If you cannot get a group moderator, then it helps to set breaks at odds times, such as five minutes after the hour. It also helps to establish right up front how you will signal them that it is time to come back in and how you will get started. If I find myself in a large group without a group moderator, a simple technique is to tell the group, “When you return from breaks and you see a hand in the air, please put your hand up too, and as soon as all hands are up that will signal it’s time to restart the session.” This causes people in the audience to look around at those who are still talking and induce your audience to quiet each other down, rather than have you play that role.
PS: Take a look at the Managing Your Strategic Message training to learn more about presenting.