draw the main gesture of the figure.
start adding the mass of the larger body parts
continue adding parts.
look for important structural parts, such as the rib cage
an indication of where the hips are,
looking for shadows will help you refine the figure and make it look 3D
continue making hatch marks to refine the shadow areas.
the more variety you see in the shadows, the more convincing your drawing will be
an animated GIF of the above sequence.
Here is a post about creating volume with hatching and contours using a gel pen and brush that is an extension of this same technique with a pencil.
And another post about using the gel pen and brush for objects other than the figure is found on the general drawing blog.
This link will bring you to a video that demonstrates a stylized way to create volume in a figure with simple spirals.
1. The top row of simple objects is shaded with hatching; straight lines create a sense of shadow.
The bottom row of simple objects is shaded with contours; essentially hatching that follows the surface contour of the object.
2. Top row: cross hatches are hatch marks going across the original hatch marks. Vary the placement to coincide with darker areas of the objects.
Bottom row: cross contours, like cross hatching, only continuing to follow surface contours of an object.
3. Continue working cross hatching and cross contours to develop more complex shadow areas.