The best advice I could give to someone who wants to communicate better is to READ. This will improve your communication skills, vocabulary and if you make a habit of reading literature of all different levels and from different parts of the world, you will learn a more diversified sense of history and will be more able to relate to a more diverse range of people.
As far as being persuasive, you definitely want to know what you’re talking about and be able to answer any question in opposition to your point. Don’t make the mistake most political pundits make which is not listening to the opposition. If your opponent or audience says something in opposition or has a question, ACTUALLY LISTEN TO IT, consider it; first for the basic reason that they might have a point and you might learn something. Second, you will endear yourself to your listener. They will think, ‘okay, this person is not up there just extolling his opinion without thinking of how we feel.’ You want your audience/listener to know that you 1) want to make a point that you believe in, 2) you understand or you want to understand their point of view, where they are coming from, 3) you are not trying to convince them for the sake of convincing them; you actually have a point because you are a thoughtful person and your point must be relevant to them/the world.
The best advice that one can give a person who wants to become a better communicator is simply to tell them that they MUST make the effort to connect with their listeners. THEN, they have to provide evidence that shows that what they are proposing is for the common good, and finally they must commit to show how their proposal actually worked.
In order to convince someone, you must present a rationale, then you have to show what steps are to be taken to make it happen. After that you must show that the steps to be taken are viable, cost-effective, and reasonably within the parameters of the institution. In the end, you have to demonstrate in qualitative or quantitative means that the plan worked, and that there are many ways to prove it.
As long as you keep a goal in mind, you will be clear enough to present a scenario that is reasonable and understandable. Persuasion is as much about convincing as it is about proving. Hence, be ready to convince, but also be ready to back your statements with enough evidence that will not throw your point of argument away.
I believe that to be a persuasive communicator, you need to be totally convinced with whatever subject you are talking about. You won't be able to persuade people to do something, buy something or even made them consider whatever you are telling them unless they feel it through your words. Being a persuasive communicator requires a person with strong personality, someone who can have influence over people and knows quite well how to do it. Think of it, do all sales representatives are totally convinced with what they are selling?? I doubt. Many of them need a job and they need the money to carry on living. So, how do they manage to covince people to buy things, they themselves don't believe in and would never buy? It's so simple. They train themselves to do so. They get enough knowledge about whatever they are selling and would always be able to answer customers' questions and in time they get used to do so, easily and without much effort.