Symmetry and Contrast

Mark Kennedy is a great resource on all things animation and He Covers This Very well >>
This will be repeating what he says

when you are trying to create good art symmetry is not usually your friend. Symmetry flattens things out and makes them uninteresting. Contrast is desireable for visual interest and symmetry is the opposite of contrast.

We don't want both sides of a figure to be symmetrical. We need to find variety in our poses so that each side is different. Here are a couple of simple tricks to help you do this.

Be aware that it switches back and forth from side to side in most drawings (like it does in both of these bird poses). Below, see how the spine is a simple straight and the belly shows the structure, then in the head it switches: The underside of the beak is the gesture line and the top of the head is the structure side:

One side of the figure is a simple line. It's the line of action, it contains the gesture and the force of the pose (usually it's the side with the spine). The other side is a more complex line and it contains the other stuff that makes the drawing look like it has volume: it has all the anatomy, the breaks in the clothes, etc.

This also creates a cool visual contrast: there's a simple side and a complex side. That creates a contrast that's pleasing to the eye. 

A figure with two simple sides looks too plain and uninteresting. 

A figure with two complex sides lacks force and has an unclear gesture.