Persuasive speaking unlike general speaking is not about simply trying to get your point across, it's about trying to get your audience to think and feel a certain way so that they take a required action.
Think of it this way, when you see a politician on stage talking to you about why you should elect them; they are trying to convince you to take a certain action.
They'll use their words, they'll use their influence, and they'll tell you how electing them will benefit you.
You can use this same approach when you are speaking in a public platform; here are a few ways to be more persuasive in your speaking.
You must understand the power of words: There are certain words you can use to get your audience to take heed immediately.
By using these words you'll automatically have their attention.
Of course there will be different circumstances for different people; they all will have different triggers that'll get them focused on what you're saying.
Copywriters use this approach all the time, there is no reason you can't use these same words to push those triggers.
For instance if you're trying to sell something using words like "guaranteed" or "secret" will make people pay attention.
You are telling them that they need not doubt and that what they are getting is something special that the general public doesn't have access to.
If you are just talking to get the crowd to do general stuff like donating money, contacting somebody, convincing them you know what you're talking about, or even just moving them emotionally you can use similar trigger points to make them take the desired action.
You must talk as if you believe what you're saying: Did you know the second that you doubt your own words other people will know it because it will show in your facial expression and your body language.
It's been said that most of what we communicate is communicated often before we ever open our mouths.
If you don't trust or believe what you're saying you'll have a hard time delivering a persuasive speech.
You must show conviction; you must walk the walk even if you are nervous.
You have nothing to be nervous about.
Presentation is everything: Presentation will affect how people view you, but I don't mean simply dressing well and being proper, I mean the way you deliver your speech.
If you do it correctly people will have no reason to doubt that you know your stuff.
If you are not set up right or it looks like you did not put enough energy into it people will notice and you'll begin to lose them.
You don't want this to happen, so make sure you deliver your speech well and with regard to your audience.
Getting people to believe your words or to be inspired enough that they'll take some desired action is truly a skill you should have if you want to be affective in your public speaking functions.
Using the three principals I outlined above will assist you in sharpening that skill.