How you speak is as important as what you speak

How you say something carries as much weight as what you say.

If you are disorganized or offensive or even too loud or too soft, your listener will not concentrate on what you are saying. They're spending too much time thinking about how you are speaking.

How you speak covers a multitude of topics, but it easier to break the elements down to:

• Structure - How you organize what you say
• Grammar - How you word what you say 
• Delivery - How you say it, including eye contact, voice control and body language

Structure

The structure of what you are saying starts with one simple thing: What's your point?

If you start speaking and you don't know what your point is, why you are speaking? In most contexts you really needn't bother to start yammering simply to add your bray to the fray.

Now this doesn't preclude speaking to move along a conversation that's just for fun. That is a point in and of itself. Even in a pleasant conversation, if you simply repeat what the previous person said or repeat yourself, you will not add to the conversation and you'll be left out or the conversation will simply wither away.

Participating in a pleasant conversation is a different reason for speaking than trying to convince your boss to give you a raise. Having a different point is going to change the structure of what you say and this is a key example of it.

When your point is "I'm important to this company and I would like a raise", your overall structure might come down to:

1. I have made valuable contributions to the company this year - don't you agree?
2. I haven't had a raise recently.
3. Please give me a raise.
4. I know you think you can't give me a raise because of X, but you should because of Y.
5. Thanks for the raise.

Now this isn't a huge mental effort, but having a structure means having a plan and means having thought out what you are doing and why you are speaking. A good place to start. 

The how of speaking is separate from the what. This is actually a topic that has been thought on. studied and written about for thousands of years. It is called Rehetoric. This is often used in a pejorative sense - that rhetoric obscures the true meaning of speech as "in insincere or grandiloquent language".

Now though, you have an idea of the importance and value of Rhetoric - properly the study of writing or speaking as a means of communication or persuasion and how important it is each time you open your mind through your mouth. As such, here is a good introduction, a Rhetoricae.