Developing a complete range of light and shade on a figure drawing can be a daunting task when you are working under time constraints. One technique I love is actually starting on toned paper.
By beginning on a medium grey or tan surface, I can add both highlights and shadows to the form, creating a sculptural feel to the model. On a light grey surface my favorite highlight is actually a very soft blue. I've also used light purple which gives a cool effect.
It's important to remember to slowly develop your lights and darks - saving the brightest and deepest spots until the very end. Also, always consider the whole figure at one time. I have been in many situations where I made one part of the figure look fantastic, only to realize it didn't match the rest of the body!