Doodles and Finished Pieces

[Chutzpar]

Try to break the figure into large, solid shapes of tone, rather than thinking too much about the contour. Use the edge of the charcoal to block in large areas.
If the edges of the form are tonally sort of similar to the background, throw in some colour or another tone in the negative space.

Also, don't be afraid to let the paper show through! Working on tinted paper means you basically already have a midtone; use that as your base, blocking the core shadow in with charcoal and only adding subtle highlights with the chalk.

Also I can't say for certain without seeing the model, but it looks to me like you're idealising the figure somewhat. This sounds obvious, but draw what you actually see, don't allow yourself to rest too heavily on assumptions about what body parts "should" look like.