I stated earlier "After the drawing is complete, students can work back into the objects to further show the value in the still life with hatching and cross-hatching marks. Again, this will help them to see value. Emphasize that their lines be neat and organized or the value in their work will not be believable. I tell them that every line must represent something in contour and it also applies to the hatching/cross-hatching."
I should have added that I also emphasize line as direction which would go with Judie's post for cross-contour line. In the hatching marks students can decide how they want the eye to be led through each object - up and down, side-to-side, etc. If the object is curved then their first hatching lines should be curved to demonstrate the volume/form of the object. Thus, their hatching lines are really a pre-curser to structural lines which they will learn with me at the AP level.