Stage presence is surely a rare skill, and one I have trouble teaching to my music students, though I've no problem with it myself. It seems that one has it or one doesn't, and I am not clever enough to figure out how one gets from not to have. Have you any suggestions?
It seems to me that the primary trouble with having no stage presence is an excess of self-consciousness, as opposed to self-awareness. What I noticed about my amateur hat models is that they became, somehow, less themselves while modelling; some essential essence vacated their bodies, while they meanwhile walked too fast, looked at nothing in particular and jiggled around nervously.
Good models, on the other hand, are grounded. They have an intuitive understanding that it is not the pose or the clothes or the activity which matters; it's them. This may seem egoistic, but it doesn't have to be--remember, "you are a child of the universe, no less than the trees and the stars; you have a right to be here."
(That's from Desiderata. I have had it on my bathroom wall since childhood. I will discuss more of it later.)
In any case, when attempting to teach stage presence to those who lack it, you need to get them to be in their bodies while on stage. You might do this by putting them on stage when they are required, specifically, to do nothing at all.
Start, perhaps, by having them lie down on a darkened stage with their eyes closed. Play "Fanfare for the Common Man" extremely loudly. Ask your students to understand that the earth is holding them up; ask them to feel their physical bodies weightily on the ground. Ask them to feel the music, going through them like a sounding board. Point out that when they are making music, they are the instruments; like a flute or a guitar, sound waves resonate through their actual bodies, making their mere physical presence as significant as the floorboards.
This sort of thing is, actually, not easy. It may take some time. Give it to them. When you think they might be there, have them sit up, still with eyes closed. Let them get used to sitting. Raise the lights a little; have them open their eyes. Ask them to maintain that sense of simply being physically present. After awhile, perhaps they will be ready to stand up.
Now we get to the next crucial phase of self-awareness--making eye contact. Self-conscious performers do not do this. They focus their gaze up, down, sideways, cattywampus, they blink too much--anything to avoid catching the eye of the audience and owning up to what they are doing. The psychological reason for this squirrelly sort of behavior is, I believe, a deeply entrenched belief that they are not good enough.
Coupled with this, simultaneously, is a sense of covert vanity, which urgently depends upon the audience to bestow its approval. The performer who will not make eye contact is actually an energy sink, pulling sympathetic attention from the audience instead of projecting it toward them. This is why bad performers are such agony to watch. They are literally exhausting.
To combat this, it is not enough simply to ask them to make eye contact. Most of them will be too shy to do so, and will end by staring unseeing at their audience while leaving their bodies completely. Instead, have them close their eyes, turn their backs, and imagine that they are singing to the person they love and trust best in the world, and that person alone. Again, let them get used to this sensation. Eventually you may ask them to turn around, pick an audience member at random, engage their gaze, and pour their heart out.
Above all, it is important that a stage performer learn to relax. It is always preferable to move more slowly and take up more time with any action than one might do normally. I remember, in one acting class, I was involved in a scene which required me to wake up. Intuitively I took a deep breath, opened my eyes, and for a moment I just sat, looking blankly at nothing.
My teacher was beside himself. "What was the best part? When she woke up. What was she doing? Nothing. That's so great."
Really, the populace is easily impressed.