Your words and body language communicate a lot to the person with whom you're speaking. Many times it will be saying something different than what you are trying to say. A poor vocabulary can distract from a great idea. Body language that shows impatience may tell a listener that you feel you know more than them. The better you can communicate, the easier it will to get things done and avoid misunderstandings.
Practice what you are going to say. If you are giving a speech, stand in front of a mirror so you can see how you look. It can also help with other types of talks you might have to make so you don't come across to someone as threatening if you mean to appear sympathetic, for instance.
Choose the right place for the discussion. An old saying goes, "Publicly praise, privately chastise." It's a good rule of thumb to follow. The idea is to not have the person you are trying to communicate with be distracted by where you are.
Know what you want to say. While you don't necessarily need to have a speech memorized (though in some cases it's useful), know the points you want to make. If you will be involved in a two-way conversation, knowing the key points you want to communicate will allow you to bring the conversation back on topic if it starts to wander.
Know what your goal is. Are you just trying to inform someone of something? Do you want them to take action? If you know what you want, then if you get it, you know you have communicated effectively.
Speak well. Keep your language clean. Use proper grammar and complete sentences. Don't insert a lot of "uhs" or "you knows" into your sentences. Don't mumble.
Maintain eye contact. It makes people feel you are speaking to them and not at them.
Use non-verbal communication. Gestures, facial expressions, body language can all help you make your points. Don't be afraid to use them, but use them properly. Don't swing your arms around without purpose. Use your gestures to emphasize points.
Listen. If you're talking, you're not listening, a classroom poster read. Listening to responses to your points will allow you to fine tune your presentation to be more effective.
Ask questions that show you are listening and can clarify the what you have been told so you can formulate a proper response.
Don't plead or whine. Keep humor clean so as not to offend.