A good GM has a grasp on the rules and the plot-line of the adventure.
A great GM can present the game in a manner that energizes and focuses the attention of his/her players. This skill in public speaking is invaluable.
Like any form of communication, information can be hard for the listener to follow if it isn't clear or if it becomes jumbled by white noise. In this case white noise is the words "uh" or "um".
Example from a game I played last week:
"So..um..so you guys see a statue and it's...um... two...hundred feet tall. Uh...hmmm..you can tell that...uh...the head used to have a crown... but the...uh..crown...uh.... has been worn away by time."
That's hard (at least for me) to listen to. Here's that same info again without the white noise.
"So you guys see a statue and it's two hundred feet tall. You can tell that the head used to have a crown but the crown has been worn away by time."
Yeah, I know. It's easy to type this out versus actually saying it. However I have trained myself to avoid the white noise words whenever I talk. Every day. No matter who I am talking to or what I am saying. Sure they occasionally slip out, but they don't punctuate a sentence more than once.
But the above examples should illustrate how these white noise words can suck the momentum out of your game. So what can we as GMs do about it?
The biggest step would be to enroll in some public speaking or even acting classes. If that's not an option then consider this. White noise words slip out to fill the gap between our mouth and our brain. The remedies for this are knowing what we are going to say before we say it, and confidence.
When the heroes come to the clearing of the statue, I know that I am going to give a description of the statue and this is the description. After that is clear in my head (it really only takes half a second) then I begin speaking. A lot of the white noise words are generated when a person starts talking but then stops because they haven't thought that far ahead or they don't have a clear idea of how to present the information. So don't rush yourself. Take that second or two to compose the thought before rushing in.
As for confidence, it's not just the confidence in talking in front of other people. It's the confidence in what you are saying. It's knowing that what you are saying is what needs to be said and that you will say it correctly. I know that sounds strange but it's true.
As long as you believe in what you are saying, you can deliver it clear and with an intensity that your players will pick up on and feed off of. A lack of confidence in the words will give rise to white noise words and/or you will sound bored as you read it, which is another thing that can kill player momentum.
So go forth! Practice! And believe me that this one skill will make a huge difference in how players enjoy your games.