Make the loops follow the general flow of the muscle. Knowledge of anatomy helps at this point. If you don't feel confident doing this, find a reference that will help you to make sure it flows correctly. You will also find that the pieces join together better if you follow this rule.
In Fig.22 you will see a higher density of edge loops at the corner of the mouth. This is quite important because if you were making it for an animation you would need these to make the face smile, speak or kiss. You can see how the polygons flow around the face and around the neck (Fig.23 - 25).
In Fig.26 you can see how the direction of the loop follows the collarbone. At the tip of the collarbone we will need to create a circular shape that allows for the rise in the shape of the shoulder.
Make the shoulder loop flow down the chest. This will allow the geometry to behave in a similar way to muscles after rigging (Fig.27 - 28). If you do this correctly you can get good results using simple blend shapes matching the bone's rotation.
Continue to follow the flow of the muscles and forms for the nose and eyebrows, as I have done in Fig.29 - 30. You will be doing this all over your model, matching the flow in the separate areas and then joining them together when the different areas meet (Fig.31 - 34).