Speak using specific examples. Regardless of what it is that you may be discussing, concrete examples will always help ground a listener's understanding of what you are trying to convey. This is especially true of concepts that may require some form of demonstration, such as laws of physics or properties of matter.
Explain how the things you are discussing relate to the people you are speaking with. Speakers who tend toward the top end of the ladder are frustrating, because it isn't quite clear what they are talking about or why it is relevant. To be a more effective speaker, you want to show your listeners how your opinions or beliefs impact their lives.
Teach your listener, rather than assuming he knows something. This is a common issue speakers face when working with a large amount of technical jargon. Making any assumptions about your listener will hurt your ability to speak clearly and effectively. Help listeners understand what you have to say by instructing them in the fundamental concepts of anything you are talking about. Pay attention for cues in your listener that indicate he may not be following you, and ask questions about how you can be clearer.
Substantiate your claims using evidence and proof. If you are trying to convince your listeners that something is true, offer them proof that you are correct. This could be in the form of a peer-reviewed study, a newspaper article, anecdotal evidence from a professional, or physical evidence from an object or location. Use evidence and proof to ground your statements in reality, and to offer credence to the reliability of your viewpoint.