Allow me to illustrate. At a recent Tea@Knight, we had a discussion of mysteries. The presenter sat in a far corner of the room, his shoulders hunched, his head low, and his tone rushed and mumbled. The listeners in the room sat awkwardly, looking at one another, unengaged and even whispering among themselves about other topics. When the presenter ran out of material, he essentially called out for help and I took over. I already sat in the center of the room, and it didn't take much for my low, loud voice to capture the attention of the players. Most people I know I have a lot of "presence" and "charisma," and those quickly turned the night into a productive one. But "presence" and "charisma" are meaningless words that refer to a skill at speech and rhetoric, an understanding of how to capture an audience's attention. I have no innate characteristics that help me in this regard (other than my height and the physical strength of my voice), just skill and understanding. The fumbling presenter in question could perform just as well as I did, with practice.
Rule 1: Command Attention
You can't tell a story if nobody is listening to you, period. Gamers are an unruly bunch at the best of times, quoting geeky films, chatting with one another, interrupting the game with jokes and so on. Allowed to run its course, most of these well-intentioned interruptions will ruin the game, eventually. However, if you learn to command attention properly, the distractions will fade away until you have a table of players who fixate their attention almost exclusively on you, and they'll become lost in your imaginary world space, which is exactly what you want.
Occupy the physical center of attention. If you sit on the edge of the room, out of eyeshot, then people won't know to look at you. They'll naturally look elsewhere. We've already talked about how powerful a human's sense of sight is. As they say, out of mind. Your players will find their train of thought wandering if you don't anchor it. I personally prefer to either sit in a unique chair (if everyone else is on the couch, I sit on the recliner), or at the head of the table, and I as well as several other GMs I know, prefer to stand when we really want to occupy the players' attention. Looking up at the one guy standing in the center of their field of view really holds their attention fast. Some other GMs use alternate tricks, like flashy GM screens, artwork, or video, and these work too, but be careful that they don't distract from you or the story you're trying to tell.
Your posture matters. Hunched shoulders, a stiff body and lowered head suggests that you don't want people looking at you, so they won't. Sit upright or stand tall. Lift your head and your chin slightly. Tighten your belly and straighten your spine as though balancing a book on the top of your head. In addition to improving your height, a tight belly and straight posture improves your breathing and the power of your voice. Also, such a stance suggests confidence, and a good GM must project a sense of leadership and confidence, because the players don't know what's going on and expect you to, and also because pretending to be elves in a forest is a little silly, but if the guy leading such a game doesn't blink once at the silliness, the other players will set aside their embarrassment and play more forthrightly.
Nothing grabs someone attention like eye-contact. Someone looking into the eyes of someone else is intense, but even looking at someone grabs their attention. Hiding behind your GM screen, reading off a description is basically the worst thing you can do because you're not looking at the players. Lift your eyes, look directly at the player to whom the description is most pertinent, and you'll find his eyes fixate on you in return. But don't let a few players dominate your attention (the danger of a pretty girl and a weakness of mine, I must admit). Spread that attention around. Every time a player realizes you're looking at him, he'll look back at you, and you'll recapture his attention.
So, you're standing tall, looking at your players, occupying their attention. Well done! But remember that running a game is a touch different than simply telling a story or giving a speech. Here, your audience participates with you, so you must learn to pass that attention from yourself to other players. Your actions bring the players into the game, setting the scene, and gathering all of their attention into a single place. Ideally, every player should invest their attention in you. Then, when it's someone else's turn, you have only to gesture to them, passing the baton, and all the players, as one, will turn to this other player. Your ability to gather attention becomes your ability to gather attention for someone else. Eye contact is important here too, not just to gain the attention of another player, but to see who is bored, or (more importantly) who is particularly engaged and wants to say something. If you're in the midst of a description when suddenly you notice one of the players squirming in your seat, you can stop and point to them and say "What do you want to do?"
Rule 2: Pace your speech appropriately
Once you have your player's attention, you can tell the story you want, but how you tell it shapes their perception of it. A mumbling monotone loses your players not just because they cannot hear you properly, but because a monotone fails to engage them. We must speak with vigor and emotion, and we must vary our tone. Doing so will not only engage our players better, but the pace and style of our speech can give the players additional information and manipulate their mood to better suit the tone of the scene. Music does something similar, and we'll use music as a metaphor for how one can tone and pace ones voice for best effect.
A normal pace, the one you'd use in everyday conversation, is generally informative and neutral in tone. Such a conversational tone tells your players that the information you give them is casual and not particularly important, such as discussing the weather or what one ate yesterday. This doesn't mean that it's a poor choice or that it shouldn't be used. It serves as the baseline for your story and represents common situations. A description of a homey tavern or an unimportant character (or just about anything that isn't urgent or emotionally charged) might be done in a conversational tone.
Music often uses a slow, legato (a musical term meaning flowing and without breaks, like the sort of sound one might associate with a violin) pace to emphasize tragedy or sadness. Very emotionally charged and depressing scenes should match that pacing. Sit back, take on a serious expression and then slowly, flowing describe the terrible, tragic events. The time you take to explain each painful detail resembles the slow, panning shots of a camera, lingering on each element. This pace also works very well for romantic scenes, and you'll notice many love songs have a similar pacing and tone.
Music often uses a fast, staccato (a musical term meaning sharp and short, like the snap of a drum) pace to emphasize happiness and excitement. The rapid patter suggests a bounciness. It's fun, it's quick, la la la WHEE! This pacing is ideal for comedy, and you'll notice many comedians have a rapid patter punctuated with awkward pauses. It's also good for parties, flirtation, or anything that is enjoyably exciting without relaxing the players (as a normal, conversational tone would do).
A fast, legato pace is the bread and butter of rock-and-roll, with wailing guitars, howling singers and a very quick pace. This is the tone of dramatic excitement, danger, epic drama. You describe war and duels quickly and breathlessly, with few pauses and little time for the players to stop and think. Every moment flows into the next. This is a powerful rhetorical style, especially for role-playing games as combat tends to dominate RPGs, and the worst thing most novice GMs do for battle, in my opinion, is sitting back and letting players think. Keeping up a rapid, fluid patter will enhance the sense of excitement, reminding the players that they battle for their lives.
A slow, staccato pace is very powerful, pronounced and majestic. You often hear it in national anthems or other regal songs or, in a minor key, in horror movie soundtracks. Rhetorically, slow, pronounced words punctuated with pauses emphasize every word, like the Simpson's Comic Book Guy ("Best. Example. Ever."). You can use it to simply emphasize what you're saying, to suddenly grab the player's attention, to show them something in startling clarity, but it also builds tension. Use it when you want something to be stately or horrifying.
Real mastery comes not just from understanding these five paces, but using them in conjunction with one another to create a narrative not just with words, but with the pace of your voice. A scene begins in a conversational tone as you describe the circumstances, casually and at a normal pace, when suddenly! Enemies attack, a battle described in a a faster, legato manner that keeps the players on their tone, words flowing together as you rapidly string them together. But the players are winning! You describe their victory in short bursts! They're happy! They're going to survive! Then... horror... of... horrors! A valued NPC... struck down... and your words slow, punctuate each moment, in a staccato manner, as though the battle itself slows... as though the players... gain clarity. And then... you slowly blend your words together as the players bow their head to respectfully send their ally off to the great beyond, a terrible loss for everyone involved.
Likewise, using the wrong pacing for the scene can be interesting too. An NPC who uses a clinical, conversational tone to describe a murder instantly implies to the players that he's crazy and that he doesn't see murder the same way he does. Describing something utterly mudane (like button collecting) in a slow and stately manner to exaggerate its importance can suggest that someone takes something a little too seriously, and so on.
Rule 3: Gesture for emphasis
People don't sit around, arms at their side, face rigidly forward telling their story. Even if you do all of the above, unless you move, unless you prove to your players that you're alive, your story will come across as stiff and unreal, just like you do. People like motion. We're conditioned to react to it. It attracts our attention. Moreover, people naturally move when they speak. They nod, they smile, they wave their hands around. You need to do the same.
Gestures, by and large, break down into two broad categories. First, you have the descriptive gesture. A descriptive gesture shows the audience what you mean. When a guy crudely describes a shapely woman, his hands outline her figure, for example, or when someone is describing a friend's tendency to drink too much, he might mime a drinking motion with his hand. Of the two forms of gestures, this is the most important for role-playing. You need to show your players how things look, or where they lie in respect to their characters by pointing our outlining. You can also show characters how the NPCs react by imitating their expressions and actions. A shy girl would huddle up and bring her hands to her face, so you can do the same. A big, dumb barbarian would sprawl out with a big sloppy grin on his face, so you can do the same. Show the players your world through your hands.
The second kind of gestures emphasizes what you say. These are abstract gestures not meant to show the players something, but to attract their attention and add a little something to what you're saying. When you're using a legato pace, keep your hands low and roll them, like you're unspooling your speech. When you're using a staccato pace, point and jab to emphasize your points and drive them home. You don't need to constantly do this, only at the moments that matter the most, since the motion attracts your audiences attention.
Be careful that you don't distract from what you're saying. If you point in a direction, players will tend to look in that direction... which might mean they're not looking at you anymore. You might want that if you want to emphasize the beauty of a grand scene, or if you want to draw the player's attention away from you and towards another player. Likewise, being excessively animated might make players wonder if you're nervous, and they'll begin to notice the gestures rather than focus on the words they're meant to emphasize. Use gestures when you need them. Don't be afraid to neglect them during scenes or moments that demand emphasis.
Gestures should come naturally. People use gestures when speaking with their friends. People tend to lose them when giving speeches because they are nervous. Stagefright is a natural response to being put on the spot, which a GM is, but you have to set it aside. I often suggest that people plan extensively not because they need to plan, but because I know such planning tends to relieve nerves, and a relaxed GM is one that uses gestures effectively. More than anything, focus on relaxing and growing comfortable at the front of the table, so your gestures come naturally.
If you watch me when I run a game or when I give a talk, you'll note I use these three rules, often without noticing it. Most skilled GMs do. Practice them, and you'll have the same "charisma" or "presence" that I do: the ability to keep your players' attention and draw them into a world, not just with what you're saying, but how you're saying it.