It seems to me an often overlooked compositional element, leastwise in terms of glamour photography, is the use of negative space.
In glamour photography, an easy way to look at it is like this: positive space is that space occupied by the model and negative space is everything else in your frame apart from the model.
Sometimes, the negative space is occupied by a whole bunch of stuff. Other times, it's occupied by a whole bunch of nothing. (As illustrated by the photo I've posted on the left.)
Finding an effective balance between the positive and negative space in your frame can be one of the most powerful elements in your photo apart from the model herself. When you're framing or cropping with an obvious nod to the Rule of Thirds, for instance, you're often using composition which heightens interest in the image and a big part of how you're doing that is by manipulating the photo's positive and negative space, i.e., doing so using elements of the Rule of Thirds.
As weird as it sounds, when it comes to negative space a whole lot of nothing, or lots of nothing that is particularly interesting on its own, can add much power and value to an image. Go figure.
Course, there are times when the over-use of negative space might not be the best way to go. Like with most all the elements of composition, using negative space calls on your personal sense of aesthetics. Sometimes it works, sometimes it doesn't. Hopefully, as with all the elements of composition, you're able to recognize when and how they work effectively and when they don't. That's called having a good eye for composition.
The pretty girl at the top is Ally. I just shot Ally a few hours before writing this update. She was a terrific model who could bust some good moves and pour on the expressions and attitudes. Obviously, when cropping the image I exaggerated the use of negative space and, yeah, I did so specifically for this update. Maybe it's just me but I think it works.