There are no lines in nature (other than veins and cracks). Therefore a painting is about shapes revealing themselves in relation to other shapes. Drawings is what we use to outline shapes, what we call their 'mass'. When you look to paint a subject, note how the shapes, shadows of the objects form around a shape and how they conspire to reveal it. Shadows of an object behind a foreground object will push it forward even as the shadow of the foreground shape releases it from the floor on which it sits.
So, the next time you draw something, think about the shapes, shadows and masses that conspire to reveal the object. Even if there is some lost edges from having the same tonal values between two overlapping objects, it's up to you to let them blend or to conspire to push the shape in front forward by changing one of the object's value.
