Know your audience, become a good listener, and be mindful of nonverbal communication. These three steps will make you a more effective communicator.
Knowing your audience is always necessary when speaking to a person or addressing a group. Why are they listening to you? How old are they? What is the function and purpose of the gathering? Answering these questions will clue you in on how you should speak to them, always keeping in mind your message that you intend to convey.
Listening is critical in all communication, even public speaking. If the audience or other members of a conversation don't believe you are listening to them, you'll come across as detached, perhaps even self-important and arrogant. Listening also helps you focus and sharpen your message for particular audiences and ears. Even in public speaking, you can 'hear' the audience's mood through their nonverbal communication while you talk yourself.
Your own nonverbal communication is essential. People listening to you should see that you are speaking with clarity and intelligence, and saying things you genuinely believe. Your body can sell the words your mouth states audibly. Even over phone conversations, mood and body language are decipherable.
Work on these three aspects of communication, and you can be more effective in most settings. This applies equally to personal and professional circumstances.