I teach an advanced life drawing class this term. The students are great and the term has been going well. One of the components of an art class is doing demonstrations - demos - of techniques, processes and/or to illustrate new ways of seeing the figure, through form, gesture, anatomy or the play of light. I'm sure that everyone is different, but when the demos go well then I have more fun and the class seems to go better. Of course that may be in part that a bad demo has the effect of frustrating everyone, including me.
I usually don't hold on to demo images. When they work I tend to give them to the student who asked the question as a visual for reference. But I'm told that they can be useful down the line so I've started taking pics with my phone or when I'm having work shot. Here is a little slideshow of some recent demo images. If you have a question and I can demo a drawing or painting for it, I'll make that a future post.
Most are 2 or 5 minute drawings, with a quick overlay to show how to block in the skeleton done in a few minutes over a very quick drawing. The media is mostly Cretacolor XL sticks on paper, with a little NuPastel on better stock from time to time. I'll add a few recent head studies when I get images.