First, know what you're going to say. It's all very well and good in poetry or prose to write (or speak) whatever comes to mind, but it makes expressing yourself to others very difficult, because you will tend to jump around topics. Know what you're talking about.
After you know what you want to say, be sure you stick to it. If you want to say someone is a wonderful person, say so-and-so is a wonderful person. Don't qualify it with something else. "So-and-so is annoying, but they're still a wonderful person." What you want to focus on is the wonderful person part, but THEY are going to focus on the annoying part, and assume that the other is just to make them feel better. And it won't. If you need more to say, just think of WHY, and include some of those things. "So-and-so is a wonderful person BECAUSE they are kind and always thinking of others."
Don't repeat yourself over and over. Say what you mean, say it well, and let your reader/listener take it. Don't shove it into their brains, because they'll just ignore it. If you're trying to force feed it to them, that means that you need their approval to lend it credibility, and they'll be less likely to believe it. Say it and move on.
Don't expect people to agree with you. Sometimes they won't. Just be ready to back up your ideas, and show why you feel or believe what you say. You wouldn't expect someone to trust your answer to a math problem if they can't see how you found the answer, would you?
Know who you're talking to, and adapt what you say to fit the circumstances. Some people have their own preset ideas about things, and you won't be able to change their minds. A lot of times it's just a matter of placing a spark of idea into them, and letting them run with it themselves. Don't look for an argument.
DON'T ATTACK OTHER PEOPLE!!!! Not only is it bad form, it's just plain dumb. If you don't like an idea, focus on the idea, not the person who has it. Thoughts come and go, and tomorrow they may not believe the same thing.
Having language skills helps. Know how to structure your sentences to make them understandable. Don't use big words if you don't have to. Don't interrupt yourself.
I may add some more to this later. Please feel free to do the same.