Invite participants to introduce themselves and tell you the takeaway value they'd like to get from your seminar. This exercise reduces the level of anonymity that often results in nonparticipation and "tuning out." If you know their names, you could likely call on them, which translates into an incentive to pay attention. In addition, the responses they give --- which you should be writing down --- tell you where their interests are and allow you to tailor your presentation content to address those concerns.
Create audio-visual content that supplements your lecture material in an exciting way. Examples include videos, photographs, cartoons, graphs, maps and pie charts. Reinforce the principles you are imparting with short, simple sentences that appear on individual title cards or PowerPoint slides. This gives your participants time to write down each sentence without missing anything pertinent.
Assign in-class analytical or writing activities that can be done by individuals or small breakout groups. Divide participants into debate teams. Have them script and perform hypothetical situations. Award small prizes for the best ideas that come out of brainstorming games. Ask questions that invite participants to share their experiences with the issues in discussion. The more participation you generate through interactive exercises, the more ownership your participants will have in a successful outcome of the seminar.
Prepare handout materials such as articles, templates, reference guides and recommended reading lists. Unless your participants will need these items during the seminar, however, hold off on distributing them until after your presentation. Otherwise, they will likely read them and rustle pages while you're talking and not be as engaged.