You can be strict from the start of your presentation to manage the expectations of your audience. Tell them what you need from them and how they can best benefit from your presentation. If someone is distracted by his phone or laptop during your presentation there are various ways on how best to deal with this.
The book that is part of the public speaking kit includes various tips and tricks on how to get it right from the beginning.
Even what you do before you start your presentation can make a big impact on the actual presentation.