Sigmund Freud wrote: 

"The most favorable condition for comic pleasure is a generally happy disposition in which one is in the mood for laughter. In happy toxic states almost everything seems comic. We laugh at the expectation of laughing, at the appearance of one who is presenting the comic material (sometimes even before he [she] attempts to make us laugh), and finally, we laugh at the recollection of having laughed."

This concept has been termed "in fun" by people that study public speaking humor. If you want your audience to laugh, they must be in fun. You, the speaker, must be in fun. The emcee or program coordinator must be in fun. The whole public speaking program should be designed in fun.

Don't do anything to take them out of in fun. Don't speak about controversial subjects like religion or politics and don't make unfriendly comments to audience members. If a problem occurs which must be dealt with, find an in fun way of doing so. For instance, if I'm at a public speaking engagement and someone asks me who I voted for I say, "I voted for the USA." That's a cute way to say that I really don't want to talk about it and that my voting selection was none of his business.

Retired National Speakers Association member and one of the greatest humorists of all time Dr. Charles Jarvis, told me about a friend of his who was excellent at public speaking, but lost his audience when he forced someone to turn off a tape recorder. He was so nasty about the way he said it that the in fun audience totally turned against him.

An in fun audience is more critical for the public speaker who is there to entertain, but the concept should be in the back of every speakers mind. Your material may be controversial by nature, but that doesn't mean that you should go out of your way to do or say things that will take the audience further out of in fun.

Also, pay close attention to the total public speaking program. One friend of mine had to present comical material just after a passionate plea went out to the audience to collect funds for starving babies. He came on stage just after the teary-eyed audience had seen slides of emaciated children. If you ever get caught in this situation, DON'T start right in with your humorous material. Start out gently with a sincere reference to what the audience has just seen. Cut most of your early public speaking humor and get to your subject to ease the audience's transition to your more lighthearted topic.

How do you put in fun into practice? One time I had a ventriloquist introduce me at an early morning public speaking meeting to wake up everyone and get them in fun. You could pass out fun snacks to the audience or put balloons on their chairs. Public  announcements and agendas can be decorated with cartoon characters. Funny props are great for putting people in fun. Do anything you can to be sure your audience knows that it's OK to laugh.