The main object of the speech/lesson is to get your points across but there is a hidden agenda which the audience should not be aware of. The object of the speaker's technique is:
From this it can be seen that a good opening is essential and it MUST contain an incentive to make the audience want to listen. An example might be: "The next lesson is on forklift truck stability. This is an important subject because many forklift operators have been killed over the years when a forklift fell on them".
I'm not suggesting that you need to frighten the audience. Another incentive might be, "Good forklift operators save their employer money, some of which can be spent on ...... etc etc."
Use an incentive that is relevant to the situation, lesson or employer etc.
Getting yes responses simply means trying to get the audience to agree with you all through the lesson. Using phrases such as "won't we?", "don't we?", "haven't we?" which will help to get agreement. An example might be, "We don't want to damage our employer's loads do we?" Very few people are going to disagree! It's difficult for the listener to argue the point later once they have already agreed with it!
There are also techniques that can be used to keep the trainees listening covered on the next slide.