Try to start on time even if some of the audience is late. Waiting too long undermines your confidence, and the audience's respect for you.
The average attention span of an average listener is between five and ten minutes for any single unbroken subject. Staying too long on the same subject in the same mode of delivery will send people into the MEGO state (My Eyes Glaze Over).
Break up the content so that no single item takes longer than a few minutes, and between each item try to inject something amusing, amazing, remarkable or spicy - a picture, a quote, a bit of audience interaction - anything to break it up and keep people attentive.
Using a variety of media and movement will maintain maximum interest.
Think of it like this - the audience can be stimulated via several senses - not just audio and visual (listening and watching) - consider including content and activity which addresses the other senses too - touch certainly - taste maybe, smell maybe - anything's possible if you use your imagination.
You can stimulate other things in your audience besides the usual 'senses'.
You can use content and activities to stimulate feelings, emotions, memories, and even physical movement.
Simply asking the audience to stand up, or snap their fingers, or blink their eyes (assuming you give them a good reason for doing so) immediately stimulates physical awareness and involvement.
Quotes are a wonderful and easy way to stimulate emotions and feelings, and of course quotes can be used to illustrate and emphasize just about any point or concept you can imagine.
So don't just speak at people. Give them a variety of content and different methods of delivery - and activities too if possible.
Passing several props or samples around is also a great way to stimulate physical activity and involvement. Some trainers of public speaking warn that passing props around can cause a loss of control or chaos. This is true, and I argue that it's good. It's far better to keep people active and engaged, even if it all needs a little additional control. Better to have an audience slightly chaotic than bored to death.
Planned chaos is actually a wonderful way to keep people involved and enjoying themselves. Clap your hands a couple of times and say calmly "Okay now - let's crack on," or something similarly confident and un-phased, and you will be back in control, with the audience refreshed for another 5-10 minutes.