A small lesson that I really like comes all the way from Edgar Degas the gist of which is, "I know I should always draw the figure from the ground up but I always forget to"

The usefulness of this advice ensures the center of gravity on the figure to always be properly placed, As you build from feet to head your inclination will be to place the "bricks" correctly on top of one another. Drawing from top down allows gravity to take your hand toward two tendencies. 1.) to pull to the right as it descends and 2.) to speed downward a little too fast, elongating the figure in sections.

When I draw fast poses from a model I try to draw from the feet up. Somehow it's easier to get the figures in correct scale to each other and they all stand balanced above their own c-of-g

There's another advantage to working from the bottom up, especially for drawing in public. When that triangle shape between the legs is established, the only thing left is to find the thickness of the legs and there you have established the scale of the figure in the drawing AND you have prevented it "growing" off the page. ALSO, you will have gotten the whole figure in before the person you're drawing notices you're looking at them, and by that time it doesn't matter.

I used to draw the head first because, "if the person walked away, at least I had the head." So I became very good at getting the head. However, if you're going back and forth from the paper to the head, that person is likely going to see you are drawing them and they will react. Start at their shoes and work your way up and they don't care; in fact, they will have no idea what you're doing.