This same technique can be very effective in ending a mostly humorous presentation. Have them laughing all along while you make your points. Then finish seriously. This contrast will create a great impact. It will convey the fact that you believe in a lighthearted approach to the subject, but the results are very serious to you.
Next issue learn how to make a point with humor.
When I recruited Tom Antion to play for the West Virginia University Mountaineers in the early 70s, he was an exciting high school football player. It doesn't surprise me that he's turned into such an exciting professional speaker. I use his tips in my own speaking engagements. I make no outbound cold calls whatsoever. I can not even stand the thought of it.