In any typical presentation, people are most engaged during the first 10-15 minutes and then attention span drops off significantly.
Key is to keep people engaged every 4-5 minutes. Ask questions. Elicit responses from audiences. Add some humor.
You should use the tell - show - tell technique. It allows you to get their attention and the presentation should be subdivided in subthemes, each one with some starting point, getting then to the middle of the subtheme and then ending with what you just learned from what you showed. I would suggest this technique because it allows you to get all the attention in all points of you presentation. You also have to be dynamic and work with your voice tone, this is also important. As Brian said, also interact with the audience, asking questions and adding some humor.