The punch line gets its name from the delivery technique used. You must punch the line out a little harder and with a slightly different voice than the rest of the joke. Lean into the microphone and say it louder and more clearly than you said the setup lines. If the audience does not hear the punch line, they are not going to laugh. Just before the punch line you should pause slightly(see Timing article on this website)to emphasize and draw special attention to the line.
After you deliver the line, don't utter another sound. Give the audience a chance to laugh. Words or phrases appended to the climax tend to delay or impede laughter. Until you get some experience, it is really tough to wait. Beginners tend to be afraid that no laughter will come, so they keep going. If you keep talking during this period, you will easily squelch the laughter. As your confidence builds, pausing will become easier and easier. Sometimes waiting the audience out will actually give them a cue to laugh even if the joke wasn't that great.
Deliver the line to one person
When you deliver your punch line, deliver it to one person and one person only. It doesn't matter how large the crowd is, you can look one person right in the eye and deliver your line.
The person to whom you deliver the punch line is NOT randomly chosen. I deliver punch lines to a person I know is going to laugh. How do I know? I pay attention. That's how I know. It all starts with my pre-program research. If I have spoken to any of the audience members and they were laughing with me on the phone, I'll seek them out before the program so I know where they are sitting. That way I can look directly at them during the program. Before the program starts, I mingle with the participants, not only to meet them, but to see who is and who is not in fun (mingling with them helps to put themin fun (see the "in fun" article on this website).
In addition, I watch the audience when the emcee or program coordinator is talking. This gives me a mental note of the people who are not only having fun, but also paying close attention to the person speaking.
Watch out for alcohol
Don't be fooled by an audience who appears to be having great fun. It could very likely have been induced by alcohol at their social hour. They may be oblivious to what's happening on-stage.
After you have begun your presentation, another way to tell who to deliver to is by closely watching the audience. Some audience members who are really in tune with what you are saying will nod their head gently in approval. You should have great success delivering to these people.
Why deliver to the laughers?
There are two reasons for delivering your punch line to someone you know will laugh. The most important is that you want that person to be a good example for the rest of the audience. If you direct a punch line or comment to a person in the audience, the other members of the audience will naturally look in that direction. If they see someone laughing, there is a high probability they will laugh too. If you deliver your line to some sourpuss that hasn't laughed for 20 years, the rest of the audience will see an example of someone NOT laughing and they will be negatively influenced.
A 1976 study by Antony Chapman and D. S. Wright supports the notion that the lack of laughter or inappropriate laughter (the kind of laughter you would get if you pick on someone or some group inappropriately and they laugh to save face) are inhibitors of laughter.
The second reason for delivering your punch line to someone you know will laugh has to do with confidence. There is little chance that you will get old sourpuss to laugh no matter what you do. If you kill yourself trying and fail, as you probably will, it will knock your confidence level and affect the rest of your performance. Combine this with the fact that you will be ignoring the rest of the audience, who will be watching this person not laugh, and you'll be quickly swinging in the wind. Deliver to the ones that appreciate you!