Presentations can benefit from the beginning with a formal agenda listing the points to be covered. Studies show that people actually remember the first point and the last point better than the others in the middle; jump right in with the most important message first! Then, maybe you can save the second most important thing for your big finish.
Watch for non-verbal clues to read the audience's reaction. It's a good sign when they are smiling and nodding appropriately at your good points; but if they start looking around the room or at their watches, pull them back into the talk by asking if they understand what you are saying and what they think of your points so far. Or ask a poll question, like "How many of you think..?" Another idea is to encourage the audience to picture the benefits of your idea by saying "Imagine what this can do for....," and giving them a moment to think about the ramifications. Don't be offended by those dozers out there; remember, sitting through lots of classes in a row can be difficult sometimes, so help these folks out by being as "up" as possible! Perhaps (if appropriate), you could finish with a call for action, spelling out what they should do right now.