Practice the art of listening - Listen twice as much as you speak. Listen to what other people have to say so that you understand where they’re coming from. By listening, you can be sure of what you're going to say. Hence, your opinions and suggestions will matter.
Ask questions - If something is unclear to you, shoot a question. Asking a question is not just showing inquisitiveness. It gives the impression that you're interested in the person speaking and his words. This enables you build a rapport which is important in effective communication.
Think before you speak. This will keep you from getting into a situation where you wish you didn't say words that can offend your listeners. Hurtful words can push people to turn deaf ears towards you.
Read, read, read. Each one of us has his own expertise. You cannot be an expert on everything but you can be knowledgeable about anything. How? By reading. Yes, read instead of watching TV or playing video games.
Practice, practice, practice. Do you get startled when talking to a lot of people? Don't fret because you're not the only one who feels that way. If you have an upcoming speaking engagement, be ready with your topic a month ahead and devote your time to practice. Practice speaking infront of your loved ones and ask for some feedback for improvement.