You need a favor pretty quick. So you open your conversation with a colleague by mentioning a recent favor you've done for him or you offer to do a favor in the near future. Then, and only then, do you ask him to do this favor for you. You've learned from an early age that giving first is a successful way of getting something given to you.
Reciprocation is one of the six weapons of influence described by Cialdini, Martin and Goldstein in Yes! 50 Scientifically Proven Ways to Be Persuasive. When people feel they owe you, you'll be more persuasive than if you simply rely on logic to build your case for them to act.
You can apply the influencing tactic of reciprocation to influence audiences as well as individuals. Since you're not likely to be able to offer a specific favor to each individual in an audience, you need to think more broadly. Here are three opportunities to persuade your audiences through reciprocation:
1) Give some intellectual property in your speech/presentation which has understandable value to the audience. If you have written a small book, give it to attendees. If you have a really special process visual that they can use immediately, give that to them. It's also important to increase the perception of the value of this intellectual property by incorporating it into your speech/presentation. You can do this by making time for the audience to use it during the time you're speaking to them so they experience it immediately. A recent speaker showed a visual self-assessment tool and gave the audience a few minutes to apply it to themselves. Everyone got into it and in conversations later that was one of the most remembered components.
2) Give a usable gift. So many companies give away pens or pads with their logo and phone/website on it and they just leave it on seats or throw it into a bag. That behavior diminishes the value of the gift to a "throw away." Instead, provide a worksheet for each participant and during your speech/presentation ask them to use their (branded) pen to fill in the worksheet. You have now combined intellectual property with a tangible gift, increasing the value of what you've given and strengthening their motivation to reciprocate.
3) Do a favor for the audience that isn't given directly to the audience. For example, you could offer to write a recommendation to the manager of those in attendance reporting how well everyone participated in the program. You could ask the audience to write a list of their top 3 needs, collate them and send that to a superior for action. You could serve as a representative to a group or a erson they wouldn't be able to access themselves and deliver their message.
The outcome of giving these gifts or favors in advance is that they cause our cultural norm of giving back to those who have given to us to kick in. Then when you ask them for something--a testimonial, to purchase a book or sign up for a teleseminar, many will do so out of a sense of reciprocity. If your call-to-action is to change their behavior in certain circumstances, they will try harder to do so.
When speakers make a difference to audiences, they are appreciated more and grow their own status and stature. Remember reciprocation and include it when you're intending to be persuasive.