Understand who they are and their concerns. What matters to a group of seniors will be different from what matters to a PTA group. This will require some research and will determine everything about how you prepare the presentation. Customize your materials to suit your audience. Some audiences respond better to a slide show, others to charts and graphs.
Time is of the essence.
Keep your presentation as short as possible and never, never, never go over your allotted time. Avoid being too technical. Keep in mind that in most cases, you're much closer to the issue than they are. Your presentation is likely being done to familiarize and educate a group about issues that most directly affect them. Leave it to the experts.