If you notice she prepares herself to walk away from you by turning her face away, STOP TALKING IMMEDIATLY!
Big chance she will turn her face back to you and let you finish. Why? Because keep talking to someone who shows to want to end the conversation is typical needy and even anti-social behavior which you 'll find for instance to beggars. Keep talking then is one way communicating, like ignoring the women's signal she just gave you by turning her face away from you.
Understand that almost all strangers who want her attention: sellers, beggars, AFG's, behave like that, so...if another stranger stops her, especially if you feel not completely comfortable approaching yet, you will always meet women who wants to do the same with you, like a reflex.
If you do excactly the opposite, that behavior will not fit in her needy frame she just put you in, so she will be mixed up by that and turn her face back to you again, like "Huh? What's going on here?", which gives you the nice opportunity to finish your opening and have her full attention like you deserve.
