Liquid Pencil (made by Derivan) is my new favourite medium - using it lets me draw on the same scale as my paintings, really giving me a good idea of whether to progress or not. I also find it much easier to control tone than with charcoal, and using different pencils, graphite sticks and powdered graphite along with the liquid version lets me get different textures fairly quickly.
The above three drawings were done today in under six hours - so although drawing first may seem a waste of time, it really isn't. All three show potential to become drawings - although the top one (at 18 x 27 inches) is just a wee bit too big and if it becomes a painting I think would look better at maybe 15 x 24 inches.
Presently I'm working on doing portraits of each of my models before moving on to more complicated, several person compositions - doing this seems to make it easier to get a likeness later on as well as just generally being quite fun.