Find out something about at least 3 people in the audience that is related to your topic. Bring these points out when the group seems to be flagging. Encourage them to expand on the point, be prepared to step in if they are embarrassed, you want the audience to remain on your side.
Move around, don’t stand behind something unless you are making a point that requires you to stand left centre stage and command the group. Movement, except when it is jerky or irritating, makes the audience focus and refocus on you rather like how they would respond when normally taking in information. (Just try staring at the same spot for 15 minutes and find out how motivated you are to learn more!)
Work on your endings and curiosity. Know what you will end with and try to theme your presentation so it works up to the ending. I once brought a box on stage with me and started by opening the end then putting it down again, with a “we’ll leave this ’til later”, then several times I looked inside the box until finally I drew out the contents with a flourish to emphasis my ending point.
Involve your audience, get them to share one point about the speaking topic so far; or one point they will take away to act on.
Be as excited about your topic as you would wish your audience to be. Don’t say “this is a boring bit.” Be positive!