The closer your two shapes, the easier the transition between them. If you have many complicated shapes involved in the tween, you may end up with some odd looking shapes in between. Shape Hints will help! Shape hints allow you to decide how the shapes will morph by placing hint markers at the beginning and ending shapes.
Click on the first keyframe of a shape tween. Select Modify > Shape > Add Shape Hint. A small letter will be placed on the stage. The placement of this shape is important. Place it at a spot you want to control. Then, click the next keyframe and reposition the small letter at the end where you wish the beginning to morph to. The hints guide the way the shapes morph. Keep in mind, more is not always better. You will want to test after each placement as each placement will affect the outcome.
Open our example, it is saved as AdvancedAnimation/demos/ShapeTween-hints.fla.