Here's a really interesting post from a former disney animator about overlapping and negative space. As I've mentioned before, cartoonists seem to have some great insight into issues of composition. I'm going to spare you my rambling and let Dave Pimentel do the 'splainin:

When these two ideas are grasped and put into action your sillouette values will start to pop off the page. If your sketch or production drawing isn't working you might want to check and see if the drawing is breathing; is there enough air around and within the pose to see every part of it's acting or reason for being?

When overlapping shapes are concerned I always tell the students in my class to use the items on the model to show shapes in front of other shapes. Use the waist line to show the forms and directions of the body, the neck line and collars and even existing wrinkles in th clothing to help show that the knee is in front of the thigh. Props like wrist watches or helmets or anything the person is holding can also help when it comes to overlap.